Pokemon Crystal Pokemon can only be sent to Pokemon Red, Blue, and Yellow for the Game Boy and Game Boy Color, along with Pokemon Stadium 2 for the N64. Even then, the Pokemon you're sending from Crystal cannot be holding an item, cannot be a generation II Pokemon, (for example, no sending Sentret and Spinarak to chill with your Bulbasaur in Kanto.) and it cannot have a generation II attack. (No pursuits, rollouts, Shadow Balls, or Fury Cutters allowed.)
You cannot send any Crystal Pokemon to Ruby, Sapphire, Emerald, FireRed, LeafGreen, Diamond, Pearl, Platinum, HeartGold, SoulSilver, or the soon-to-be released Black & White.
GameFreak: they're sticklers, I know...:/
EDIT: I suppose I never told HOW to do it. xD
To send a Pokemon Crystal Pokemon to R/B/Y, you must have a Link Cable, connecting two Game Boys: One with Crystal inserted, another with Red, Blue, or Yellow inserted. For Crystal, go to the Time Capsule in the top floor of a Pokemon Center, and follow the given directions from the attendant. (As a note, the Time Capsule looks like a giant oven. =P) For Red, Blue, or Yellow, just go to the generic Trade Station and do as you would with normal trading.