Here are some general tips for some of the major consoles.
Not all PS3 systems can play PS2 games
Sony has removed the backwards compatibility technology from the newer PS3 systems. The removal of this special graphics synthesizer chip, which allowed for almost perfect backwards compatibility for PS2 games was one of the major reasons PS3 has been able to offer the new systems at a very competitive price to the 360 and Wii.
The systems that do allow for backwards compatibility are the older 20GB, 60GB and 80GB systems, so if you're hoping to buy a new PS3 that is full backwards compatible, then I am afraid you are out of luck (there are still some rumors floating around that Sony may bring this function back but nothing to date has been confirmed).
To play Gamecube games on the Wii you need a Gamecube controller
To play the virtual console games from the SNES, Genesis, N64 and so on, you will need the Classic Controllers. They are about $25 and are a good investment if you are a big fan of older generation games.
Also If you are wanting to play Game Cube games on the Wii, you are going to need to have the Game Cube controllers.