Team Fortress 2 still has a decent following.  I'd also recommend a game like Demigod or League of Legends, both have huge team elements and a FPS kills/deaths mentality.

BF2/TF2 or maybe L4D2, but not MW2

Once the BF2142 patch comes out, you should play that if you liked it(I know it was a love it/hate it thing; I loved it).

Since Ill assume you have steam ie Half Life 2 deathmatch there. All you need is to download source sdk base. Then go here http://www.neotokyohq.com/

Neotokyo really good mod!

You can watch some of the videos to learn what its all about. Its my favorite source game right now! If you dont have steam... my bad...

Team Fortress 2 and LFD2. You cant go wrong with either.

Hey Battlefield 2 is a good choice... Good memories of that game.

MW2 on consoles is good. Next year we get Battlefield Bad Company 2.

ARMA 2 is a great choice! But its has a really high learning curve at first. You also need a somewhat good pc. But once you get it down... 50 player co op... omg fun time! In my opinion best military shooter out there...

Team Fortress 2 is what I've been playing more or less since it came out, and it really is that good.

Left 4 Dead 2 is cooperative, and also seems excellent.

A lot of people like Call of Duty 4: it hasn't grabbed me because I find it thematically uninteresting (as in, too serious), but the game mechanics certainly work well.

And then of course there's Counter Strike, which has been the most popular online FPS for the best part of a decade now.

Serious Sam: it's not exactly what you're asking for, but damn is it good fun coop.

funnily enough i was just thinking about my personal fps fix for the first time in months moments before open the forum.

Unreal Tournament 3

loads of game options, simple mechanincs and fun weapons and vehicles.

it's not realistic at all (hence unreal) but it's loads of fun.

Left 4 Dead 2 > everything else.

The fact that you were playing BF Heroes though screams to me Team Fortress 2.  Buy the Orange pack and you will be in FPS heaven for a while at least.

Another vote for Team Fortress 2.  So much fun, even after all this time.  I haven't played Left for Dead 2 yet, but I suspect it's also excellent.

I'm big on the Call of Duty series with the exception of MW2. There are still a lot of people playing Call of Duty 2 and Call of Duty 4, World at War is OK but not the best of the bunch. MW2 they killed the multiplayer server with the lack of dedicated servers and the inability to prevent hackers from cheating in the servers which they claimed was the reason they created the game without dedicated servers. talk about backfiring.

I hear L4D2 is awsome, I am internetless at home at the moment so can't say it the online multiplayer is good but youtube videos look great.

I hope Bad Company 2 will be good too.

Hmm... I've downloaded the ArmA 2 demo so I'll see what that's about, but I'm getting so many votes for TFC2 that I guess I'm compelled to try it. My internship just came through so I feel like I deserve something

Tribes/Tribes 2

Nexuiz. It pwns all others so far listed because of 1 beautiful thing. The GPL. Which Nexuiz (pro. NEX-EE-UZ) has been released under, making it not only FREEWARE!!!!! but also giving the source code out for other devs to play with (I haven't played with it, as right now I am fascinated with the UDK from Epic Games (like a dev version of UT3)).

The greatest online multiplayer arena-style FPS game of all time is...the original Unreal Tournament (1999) (aka UT99) which people still play online, greedily, to this day.  Yes, it's 10 years old, but it has an amazing chemistry and feel that no other game has replicated.  This game was so beloved back in the day that over 10,000 custom maps and mods were released for this game.

It has an excellent Deathmatch game (including Team Deathmatch or TDM) and also the best Capture-the-Flag game.  There are still populated regular weapons custom map CTF servers and people still play 5v5 clan-match-style pickup-games (PUG) matches.

I recommend getting the Unreal Anthlogy which will give you the original Unreal single player game that preceeded Unreal Tournament, Unreal 2 (which isn't all that great), most importantly, Unreal Tournament (1999) itself, and UT 2004 which is a good game in its own right.

As for UT 2004, it is a great game in its own right but for different reasons.  It's on-foot fragging games--DM and CTF--are inferior to those of UT99, but it shines for its vehicular game, Onslaught, which is great fun.  There's also a populare user-made mod game called Invasion Monster Mash RPG which is essentially a non-competitive shooting gallery game where servers keep track of your player name and IP so that you can play a character and level it up over time.

Anyway, if you're look for a good FPS with a Deathmatch game, you definitely must consider UT99.

Link to Unreal Anthology for $10 shipped at Newegg.com

You might also be able to purchase and download the games over Impulse.

What about UT3?  It's a sad story.  Epic Games succumbed to consolitus and also hopped into bed with Midway, a dying game company.  The end result was that UT3 ended up being a consolized abomination that was released months before it was ready and that really doesn't deserve the Unreal Tournament name.  The feel of the game and the game play isn't too bad and it's worth looking at if you can buy it for under $10.  It's not as bad now that it's been patched up, and is probably better than most FPS games on the market.  It has low online player counts and overall it's just not as much fun as the original UT99 and the UT3 Warfare game isn't as much fun as UT 2004's Onslaught (which people still play online).  The Deathmatch and CTF games aren't bad but personally I prefer the original UT99 for that.

well, I think UT3 does have some advantages over other UT titles- its still extensive modding community.

However, that is likely changing now that the UDK has been released. UDK is basically a development version of the UT3 engine (comes with some UT3 demo stuff for playing around and testing). I foresee that eventually most (if not all) UT3 mods will end up as freeware games made with UDK.