As for my game recommendations I differ from Matt and Jesse in that I really prefer the Halo series over the COD series in every mode, both single and multiplayer. There's a lot more to do, and it is much easier to play with more people. When playing online COD only lets one person play, for Halo you can play online with guests on your Xbox. I think Halo is a much better game. CoD is still worth picking up though, I'm not saying that the games are bad, Halo is just better. I could go into more detail on that if you'd like me to, but I'll just leave it at that for now.
I guess it all depends on whether you prefer your shooters to be realistic or cartoony.

I know which one I like better. (That being said, this thread actually prompted me to break out Halo 3 last night for the first time in months to
find a few more skulls unlock a few more achievements, and I'm somewhat looking forward to ODST later this year.)
I think I'm in the minority here but I hated the LEGO Star Wars game. It was kind of fun/cute for the first few times I played it, then I thought it was just too repetitive and beating it 100% took FOREVER.
I actually agree with this, somewhat. I played the **** out of the very first Lego SW game (prequel trilogy) way back when, and played through most of the second (original trilogy), so by the time I got around to going through Complete Saga several times, I had already played through most of it several times over, and was about to throw it out the window while trying to get 100%. That being said, the first playthrough is a blast and even the second playthrough is a blast, too, while trying to go back and unlock everything you weren't able to on the first. Still worth checking out though, later repetitiveness aside.
Was going to recommend the Gears series (or at least the first--haven't really played the new one), but it's not technically an FPS, so yeah.
Another one worth checking out is Crackdown. It's not in a genre that Jayson mentioned, but it's a hell of a lot of fun, anyway. And cheap now, too.
And again I'll mention Bioshock. One of the most unique, cinematic shooters I've ever played. Can't have too much love for Bioshock.