Halo: Reach, not really dead, but still hasn't even got half of the amount of players Halo 3 had, even when Halo 3 neared it's end.
Also, for some reason, I can't get on to BFBC2 on my Xbox, so that's pretty dead for me, please don't tell me you need Vietnam to play online?
Fighting games seem to be pretty dead too, aside from the new release of Mortal Kombat. I only saw around 5 games on BlazBlue: CS around a week ago, but since a new version is out, I'm assuming it's probably a bit more lively now. SSFIV sounded pretty dead, from what my friend told me earlier today.