Cut Down Grassy Areas
Once you get the HM "cut", your pokemon can cut down grassy areas. Use "cut" and it will cut a two by two area. Wild Pokemon will not attack you in the area you cut.

Duplicate Items
To get 99 of any item, first put the item you want to multiply in the sixth item slot. Now use FLY and go to Viridian City. Talk to the old man almost all the way north of the town. When he asks if you are in a hurry, say no and watch him catch a weedle.

Right after that, fly to Fuchsia city. Make your way south and, when you you reach the water's edge, use SURF and make your way to Seafoam island.

When you get to seafoam island, swim up and down the beach until you run into a Pokemon called "M" (Missingno.) Once you see him, kill him or run away. DO NOT CATCH HIM! After that, you should have multiple copies of your sixth item!

Fight Land Creatures in Water
To fight creatures that normally live on land in water, go to the water around Cinnabar Island or Seafoam Islands (The only place you can be randomly attacked while swimming.) Swim along the shore of any island and you will fight land creatures. (You have to swim on a place where the square is half land and half water.)

Instead of fighting a set group of monsters in the water, you actually fight monsters from the last place you were. So to get safari zone monsters, you need to go to the section of the safari zone with that monster, let your time run out, then go to the coast. Using this trick I have caught Kangaskhan, Scyther, and Tauros!

Get MissingNo.
To get MissingNo (a glitch) go to Cinnabar Island and go into the Pokémon Research Labratory and trade with anyone in the first room. Then leave the Lab and go to the EAST of Cinnabar Island and surf up and down on the part where the land meets the water. Eventually the battle screen will come up and the screen goes black for about 5 seconds.

Once you find him I suggest you use a Level 30 to 40 Pokemon. This is because Missing No., even at Lv. 80, has a VERY low defense rating. So get him down pretty low. Even at low he is still extremely difficult to catch. So next get a Pokemon that knows a sleep move and put him to sleep. Follow this up with a couple Ultra Balls and you should have him.

Level 100+ Pokemon
Follow these instructions to catch pokemon with really high levels.

Talk to the old man in Viridian City who shows you how to catch pokemon.
Fly directly to Cinnabar Island.
Put your strongest pokemon on your team.
You might want to save now.
Surf on the shore where the land touches water.
Keep Surfing until you run into one of these strong pokemon.
These pokemon are well above level 100. Unfortunately, they will revert back to Level 100 if they gain any experience points. Rare Candy is the only thing that raises their level.

Find Mew in red version:
151-91A-7FC
151-93A-F7E
151-9BA-5DB
Mew will be in the field west of Cerulean City.

Find Mew in blue version:
151-91A-7FC
151-93A-F7E
151-9BA-6EB
Mew will be in the field west of Cerulean City.

Unlimited life during a battle:
231-645-D9D
238-NCJ-444

Triple life during battle in red version:
BBB-BBB-BBB
10B-3DD-B24
Side effects are seeing a wild Missingno.or M. Do not catch them.

Free Masterballs at the Cerulean City market:
014-55D-F7A

Sell Pokeballs for 40,000 gold:
996-11B-E6A
Disable the code to buy as many Pokeballs as needed, then enable the code to sell.

Start with Mew:
01151ED1

Free Master Balls in any Pokemart:
01017CCF

Fight Mew in every battle:
0115D8CF 

Catch wild Mewtwos:
0183D8CF 

Balloon Pikachu:
013973D1
011364D1 

Get poison sting as second move:
01281DD0

Fight selected Pokemon in every battle:
01<00-BF>D8CF
At times, the select Pokemon will be blurred, but proceed to the battle as usual. If the Pokemon and caught and used, it will not appear blurred. The level of the Pokemon should be the level the normal Pokemon would be at if obtained without the code. Use one of the following values.

00: ???? 01: Rhydon 02: Kangaskhan 03: Nidoran Male 04: Clefairy 05: Spearow 06: Voltorb 07: Nidoking 08: Slowbro 09: Ivysaur 0A: Exeggcutor 0B: Lickitung 0C: Exeggcute 0D: Grimer 0E: Gengar 0F: Nidoran Female 10: Nidoqueen 11: Cubone 12: Rhyhorn 13: Lapras 14: Arcanine 15: Mew 16: Gyarados 17: Shellder 18: Tentacool 19: Gastly 1A: Scyther 1B: Staryu 1C: Blastoise 1D: Pinsar 1E: Tangela 1F: No Pokemon Labeled As Missingno 20: Missingno 21: Growlithe 22: Onix 23: Fearow 24: Pidgey 25: Slowpoke 26: Kadabra 27: Graveler 28: Chansey 29: Machoke 2A: Mr. Mime 2B: Hitmonlee 2C: Hitmonchan 2D: Arbok 2E: Parasect 2F: Psyduck 30: Drowzee 31: Golem 32: Missingno 33: Magmar 34: Missingno 35: Electabuzz 36: Magneton 37: Koffing 38: Missingno 39: Mankey 3A: Seel 3B: Diglett 3C: Tauros 3D: Missingno 3E: Missingno 3F: Missingno 40: Farfetch'd 41: Venonat 42: Dragonite 43: Missingno 44: Missingno 45: Missingno 46: Dodou 47: Poliwag 48: Jynx 49: Moltres 4A: Articuno 4B: Zapdos 4C: Ditto 4D: Meowth 4E: Krabby 4F: Missingno 50: Missingno 51: Missingno 52: Vulpix 53: Ninetales 54: Pikachu 55: Raichu 56: Missingno 57: Missingno 58: Dratini 59: Dragonair 5A: Kabuto 5B: Kabutops 5C: Horsea 5D: Seadra 5E: Missingno BF: The rest are glitches

5F: Missingno 60: Sandshrew 61: Sandslash 62: Omanyte 63: Omastar 64: Jigglypuff 65: Wigglytuff 66: Eevee 67: Flareon 68: Jolteon 69: Vaporeon 6A: Machop 6B: Zubat 6C: Ekans 6D: Paras 6E: Poliwhirl 6F: Poliwrath 70: Weedle 71: Kakuna 72: Beedrill 73: Missingno 74: Dodrio 75: Primeape 76: Dugtrio 77: Venomoth 78: Dewdong 79: Missingno 7A: Missingno 7B: Caterpie 7C: Metapod 7D: Butterfree 7E: Machamp 7F: Missingno 80: Golduck 81: Hypno 82: Golbat 83: Mewtwo 84: Snorlax 85: Magikarp 86: Missingno 87: Missingno 88: Muk 89: Missingno 8A: Kingler 8B: Cloyster 8C: Missingno 8D: Electrode 8E: Clefable 8F: Weezing 90: Persian 91: Marowak 92: Missingno 93: Haunter 94: Abra 95: Alakazam 96: Pidgeotto 97: Pidgeot 98: Starmie 99: Bulbasaur 9A: Venusaur 9B: Tentacruel 9C: Missingno 9D: Goldeen 9E: Seaking 9F: Missingno A0: Missingno A1: Missingno A2: Missingno A3: Ponyta A4: Rapidash A5: Rattata A6: Raticate A7: Nidorino A8: Nidorina A9: Geodude AA: Porygon AB: Aerodactyl AC: Missingno AD: Magnemite AE: Missingno AF: Missingno B0: Charmander B1: Squirtle B2: Charmeleon B3: Wartortle B4: Charizard B5: Missingno B6: Missingno B7: Missingno B8: Missingno B9: Odish BA: Gloom BB: Vileplume BC: Bellsprout BD: Weepinbell BE: Victreebel

HM and TM:
First slot:
01<C4-FA>7CCF
Second slot:
01<C4-FA>7DCF
Third slot:
01<C4-FA>7ECF
Fourth slot:
01<C4-FA>7FCF
Use one of the following values.

C4: HM01 Cut C5: HM02 Fly C6: HM03 Surf C7: HM04 Strength C8: HM05 Flash C9: TM01 Mega Punch CA: TM02 Razor Wind CB: TM03 Swords Dance CC: TM04 Whirlwind CD: TM05 Mega Kick CE: TM06 Toxic CF: TM07 Horn Drill D0: TM08 Body Slam D1: TM09 Take Down D2: TM10 Double Edge D3: TM11 Double Beam D4: TM12 Watergun D5: TM13 Ice Beam D6: TM14 Blizzard D7: TM15 Hyperbeam D8: TM16 PayDay D9: TM17 Submission DA: TM18 Counter DB: TM19 Seismic Toss DC: TM20 Rage DD: TM21 Mega Drain DE: TM22 Solar Beam DF: TM23 Dragon Rage

E0: TM24 Thunderbolt E1: TM25 Thunder E2: TM26 Earthquake E3: TM27 Fissure E4: TM28 Dig E5: TM29 Psychic E6: TM30 Teleport E7: TM31 Mimic E8: TM32 Double Team E9: TM33 Reflect EA: TM34 Bide EB: TM35 Metronome EC: TM36 Self-Destruct ED: TM37 Egg Bomb EE: TM38 Fire Blast EF: TM39 Swift F0: TM40 Skull Blast F1: TM41 Soft Boiled F2: TM42 Dream Eater F3: TM43 Sky Attack F4: TM44 Rest F5: TM45 Thunder Wave F6: TM46 Psywave F7: TM47 Exposion F8: TM48 Rock Slide F9: TM49 Tri-Attack FA: TM50 Substitute

Moves:
First move:
01<01-9D>73D1
Second move:
01<01-9D>74D1
Third move:
01<01-9D>75D1
Fourth move:
01<01-9D>76D1

47: Absorb 33: Acid 97: Acid Armor 61: Agility 85: Amnesia 3E: Aurora Beam 8C: Barrage 70: Barrier 57: Bide 14: Bind 2C: Bite 3B: Blizzard 22: Body Slam 7D: Bone Club 9B: Bonemerang 91: Bubble 3D: Bubblebeam 80: Clamp 04: Comet Punch 6D: Confuse Ray 5D: Confusion 84: Constrict A0: Conversion 44: Counter 98: Crabhammer 0F: Cut 6F: Defense Curl 5B: Dig 32: Disable 92: Dizzy Punch 18: Double Kick 68: Double Team 26: Double-Edge 03: Doubleslap 52: Dragon Rage 8A: Dream Eater 41: Drill Peck 59: Earthquake 79: Egg Bomb 34: Ember 99: Explosion 7E: Fire Blast 07: Fire Punch 53: Fire Spin 5A: Fissure 35: Flamethrower 94: Flash 13: Fly 74: Focus Energy 1F: Fury Attack 9A: Fury Swipes 89: Glare 2D: Growl 4A: Growth 0C: Guillotine 10: Gust 6A: Harden 72: Haze 1D: Head Butt 88: Hi Jump Kick 1E: Horn Attack 20: Horn Drill 38: Hydro Pump 3F: Hyper Beam 9E: Hyper Fang 5F: Hypnosis 3A: Ice Beam 08: Ice Punch 1A: Jump Kick 02: Karate Chop 86: Kinesis 8D: Leech Life 49: Leech Seed 2B: Leer 7A: Lick 71: Light Screen 8E: Lovely Kiss 43: Low Kick 60: Meditate 48: Mega Drain 19: Mega Kick 05: Mega Punch 76: Metronome

66: Mimic 7B: Minimize 77: Mirror Move 36: Mist 65: Night Shade 06: Pay Day 40: Peck 50: Petal Dance 2A: Pin Missile 8B: Poison Gas 28: Poison Sting 4D: Poison Powder 01: Pound 3C: Psybeam 5E: Psychic 95: Psywave 62: Quick Attack 63: Rage 4B: Razor Leaf 0D: Razor Wind 69: Recover 73: Reflect 9C: Rest 2E: Roar 9D: Rock Slide 58: Rock Throw 1B: Rolling Kick 1C: Sand-Attack 0A: Scratch 67: Screech 45: Seismic Toss 78: Self Destruct 9F: Sharpen 2F: Sing 82: Skull Bash 8F: Sky Attack 15: Slam A3: Slash 4F: Sleep Powder 7C: Sludge 7B: Smog 6C: Smokescreen 87: Softboiled 4C: Solar Beam 31: Sonicboom 83: Spike Cannon 96: Splash 93: Spore 17: Stomp 46: Strength 51: String Shot A5: Struggle 4E: Stun Spore 42: Submission 4A: Substitute 2A: Super Fang 30: Supersonic 39: Surf 81: Swift 0E: Swords Dance 21: Tackle 27: Tail Whip 24: Take Down 64: Teleport 25: Thrash 57: Thunder 56: Thunder Wave 55: Thunderbolt 09: Thunderpunch 54: Thundershock 5C: Toxic 90: Transform A1: Tri Attack 29: Twineedle 0B: Vicegrip 16: Vine Whip 37: Water Gun 7F: Waterfall 12: Whirlwind 11: Wing Attack 6E: Withdraw 23: Wrath

Infant first items:
01001FD3