Pokémon can be traded between LeafGreen, FireRed, Sapphire and Ruby.
Go to the far right counter and select the Trade Center option at the Pokémon Cable Club or go to the Wireless Union Room and use the wireless adapter to connect with other users. To trade Pokémon, go to the upstairs of the Pokémon Center, and have your friend go there as well (in his/her version of the game).
