My brother was checking out a review for a game recently -- I can't remember which one, but basically it was a good game with 4 player split-screen multiplayer (the most important type of multiplayer on a console

) and the review made no mention of it.
I was thoroughly disappointed, and we discussed that many games with 4 player split-screen have that feature ignored by reviewers unless that feature is the game's saving grace.
Hence my signature.
So I'm glad you brought this up.
I'm also glad you said "PS3 and Wii" because those are exactly what we've got.
To emphasise what's been said by others:
- Little Big Planet for SURE
- Super Smash Bros Brawl DEFINITELY

(and that's actually the only game we have on Wii)
Others:
- Resistance Fall of Man -- a solid first person shooter if you really have to play a FPS on a console (the multiplayer is solid). Resistance 2 was apparently very incredible, I just don't know off the top of my head if it had 4-way split screen.
- Marvel Ultimate Alliance 1 or 2 are basically ONLY good if you're going to player with a chunk of other people in the same room, and then they're great.
- Motorstorm Pacific Rift if you've got an HDTV -- track visibility can be really difficult, but it's a great split-screen racer.
- Supersonic Acrobatic Rocket-Powered Battle-Cars got bad reviews because all-in-all it's not a fantastic game. But the multi-player is WAY cool. Split-screen of course. Download the demo off PSN if you're hesitating because of its reception, but I really liked the multiplayer.
- Rock Band -- I don't actually have it but most of my mates have it on one console or another. Brilliant

Of course the best and most fun in the world is DDR -- if you've still got an old PS2 you can easily get DDR and two dance-mats (TWO is very important. Not one) for REALLY cheap on ebay or something, or just get the latest version on Wii but it'll set you back some more. The most brilliant fun that can be had in video games, I reckon.
Thanks for persevering

Jibb