Before entering Mt. Moon, go into the Poke'mon Center. There is a man, trying to sell a Magikarp for 500 Poke'mon Money. Buy it and give it to the daycare man in Cerulean City. Let him raise it up to level 49. Then take the Magikarp and raise it up until it is at level 50. It will turn into a Mew.
Getting Missingno (only Red and Blue Version [WARNING! This cheat will screw up your game!]):
Go to the man in Viridian City who asks if your in a hurry. Say "no" and watch him catch a Poke'mon. Then don't move fly to Cinnabar Island and surf up and down the right side.
Get Togepi (I tried this on red version, but it didn't work. Probably only Blue Version):
Make sure that you have the itemfinder. Then, go into Mt. Moon and use the itemfinder 100 times in a different spot. On the 101, it will say that you found an egg. Raise it up (I think) and it will hatch. Now you've got Togepi!
Fight Safari Zone Poke'mon out of Safari Zone (Red and Blue)
You go to the Safari Zone and buy the 30 Safari Balls. Then, once you're in the Safari Zone, exit it. The guy at the counter will say "Leaving early?". Say "Yes". Then, Surf to Seafoam Islands, but don't go on the land. Just Surf up and down the coast. You will find Safari Zone Poke'mon. This cheat is quite helpful if you're trying to find hard-to-catch Poke'mon like Pinsir or Tauros.