Here are some I heard, most are just completely dumb.
Red/Blue
- The Infamous Mew and the Deserted Truck
- *Insert Stone Here* on Mewtwo creates Mewthree
- Name Pikachu "Pikablu" and it would turn into a Pikablu (aka Marill)
- Kill 666 Eggxecutor and an egg would fall that hatched into Togepi
- Leaf Stone on Eevee creates Leafeon/Grasseon/Treeon
- Moon Stone on Eevee creates Lunareon
- Catch a Dratini in Red, Trade to Blue, Evolve, Trade back, Evolve, Trade back, Go to Cerulean Cave, Use Fire Stone on Dragonite and it evolves into a Yoshi.
- Name your Character "Mario" or "Luigi" and your character becomes Mario or Luigi with an Elemental (Water/Fire/Grass) Yoshi as your starter choice.
- Name your character "Link" and your character becomes Link.
- Bill's Secret Garden
Yellow
- Talk to Brock/Misty 100 times after beating the Elite 100 times to get them as companions.
- Evolve Charmander and it won't Listen to you, as in the anime.
- Bulbasaur won't Evolve.
- Squirtle can gain Squirtle Squad Shades
- You can go to Orange Islands via Bill's Backyard.
- Pikachu can learn fly if an item called "Balloons" is used on it.
- Lose to Lt. Surge's Raichu 90 times and Pikachu will opt to evolve into a Raichu (or a Pikablu/Marill)
- Beat the Elite 2,000 times and you can go to Mt. Silver, but only if connected to Gold/Silver.
Gold/Silver
- You can play a new story as Red set after Red/Blue/Yellow if you beat him 87 times in a row without leaving the area.
- If a Marill is traded to Red/Blue/Yellow, its name changes automatically to Pikablu.
- Giovanni can be found in a secret room in the Viridian City Gym if you have a Special Legendary in your party (Yoshi, Mewthree, Etc). When you find him, he fights you with a lvl 100 Armored Mewtwo, as seen in Pokemon the First Movie.
- Beat Red for a 47th time at 3:33 exactly and he will give you his Pikachu, which will follow you and can learn Surf.
- After getting Celibi, you can Time Travel to Old Kanto and go into the Safari Zone.
- After getting Celebi, you can Time Travel to Old Kanto, where you can meet Red's Mother as a trainer named Delia. Beat her 13 times and she'll ask you to marry her, thus making you Red's Father.
Crystal
- Beat Red 88 times and he'll ask for your Phone Number if you're a female. He'll call you in 17 hours and ask you on a date.