In addition to those already mentioned here are a few others.
Super Monkey Ball Deluxe - Lots of multiplayer party games.
Mashed - Insane overhead view Car racing like MicroMachines, 4 players.
Timesplitters 3 - Crap in single player, best 4 player FPS on the PS2, good 2 player co-op story mode.
Everything or Nothing: James Bond - Cool 2 player co-op mode, dodgy 4 player fighting.
Rachet & Clank 3 - 4 player, 3rd person combat.
The Bouncer - cool 4 player fighting, you need to unlock everything first.
Eyetoy Play 2 - Great multiplayer party games.
Lego Star Wars - 2 player co-op story mode.
Smash Court Tennis 2 - Best tennis game on the PS2, 4 players.
Twisted Metal:Black - 4 player car combat.
War of the Monsters - Interesting take on 2 player beat 'em up.
Onimusha Blade Warriors - Pretty good 4 player fighting.
LOTR: Return of the King - 2 player co-op story mode.
The Stuntman - This works well as a take turns party game.
Kuri Kuri Mix - 4 player co-op puzzler.
Everybody's Golf 4 - 4 Player golf.
I would 2nd the recommendations for Champions of Norrath (4 player story co-op) and Athens 2004 (multiplayer).