Off in an ocean area of Red/Blue there's this tidal divider thingy that you can swim up and down along. Weird MissingNo glitch Pokemon show up there--when you catch them, they turn into normal Pokemon, like Charmander, but their stats are weird. Boosted. You can then level them up and evolve them and get strange stats, like ridiculously high HP. I once did it too well and went above 999 and it rolled back around to low double-digit HP.
I used it to beat some jerk in a tournament when I was 12 who had a Mewtwo with Duplicate and Poison (so he'd just poison you and you'd have to break the duplicate; poison had exponential damage). It'd probably be useless these days, dunno. Christ, that was like ten years ago. Damn you kids and your flashback induction machine!