I recently imported
this. It's made of one part mass carnage and three parts aweXome (spelt with X for eXtra awesomeness). Yes, it's labelled Terra Defense Force there but the HUD calls it Earth Defense Force.
I know it's a bit older but it's total PWN. Grab a gun and blast hundreds of aliens. Even cooperative with two players. Most missions play in cities and you can destroy EVERYTHING you see there, houses, bridges, skyscrapers,... It just makes me wish they weren't restricted to the weak PS2, at times the framerate drops significantly but that doesn't happen too often. The automatic vertical aim is a bit annoying and crappy but it's not a showstopper. Homing weapons seem to always pick the target they really can't reach and give you no way to make them shoot at a specific target but even that doesn't stop the fun.
It's pretty amazing what's going on in the game all at once, you're often engaging 50 enemies at once without framerate drops yet, many enemies remain lying around as corpses for quite some time with full collision (though no ragdolls, I think it'd have to be a next-gen game for those), the collapsing buildings are some of the best looking ones I've seen, the enemies look very detailled and I rarely think these models were meant for appearing in such large numbers (i.e. low detail). Collision is pretty accurate for larger enemies (you can even stand on the surface of the smaller saucers). The sligh blur/bloom the game uses occassionally makes parts of the city look like a skybox. Until they get hit by a rocket, that is.
Video!
You can pick between a dude who can pilot vehicles (mostly useless IMO) and uses normal guns (very useful) or a girl who has a jetpack but can't take as much damage and has pretty useless weapons along with a central energy supply which all weapons drain. If you don't notice from that description, I like the dude better

.
It's not very english friendly but you'll quickly get used to what is where and what those weapon names mean (fortunately they are sorted by category which makes finding e.g. a rocket launcher easy).
Did I mention 2 player cooperative alien killing and city devastating?
Sorry for the plug but it's tons of fun, especially for a game released at a budget price. And apparently it's getting released in the UK (but not the rest of Europe) next February. Amazon has it
here (yes, that's EDF2, not the first one).
Hm, Wiki says it's getting a sequel on the XBox 360. If that comes out here I might break down and grab a 360. This game is really meant for next-gen.