Fistful of Frags is fun if you want to try your hand at the Wild West. It's not bad, but there are some elements that could use a god tweaking... For example, the ability to snipe with a shotgun.
I used to fancy myself a pretty good dynamite-tosser. Timing the fuse (which you can't see) just right so that it blows up three inches from your target's nose is so damned satisfying. But then the people who couldn't do it complained, so now there's a 1-3 second "fudge" on the fuse so you can't time it the same way anymore.
I haven't actually played the new version yet... There's a new whiskey bottle so you can heal other players (but not yourself), and I don't know how that works yet.
Empires is great fun if you happen to be on a good server. If you're not, it's junk. You just need a fast server with a lot of people on it and at least one guy on each side who actually knows what he's doing. The rest need to be able to listen.
In Empires, that's actually not that big of an order. Sure, it's harder than finding a "normal" server, but it's still there.
Age of Chivalry has problems... Lots of problems. PVKII would be alright if it weren't for the horrid player base (the "warm welcome" thread was talking about the PVKII forums).
I could never get Iron Grip to work properly... The music was always turned up to an ear-killing level, and I couldn't figure out how to tone it down.
If you want a nice twist on HL2, check out SMOD: Tactical. It's a mod of a mod (SMOD), and it has some really neat features. It's singleplayer only, however. But it is entirely possible to spawn a friendly strike team member to die for you. You can even bind it to a key.
Gmod 10 costs money, but Gmod 9 does not. Gmod 9 is wonderful, but hideously buggy. It does, however, boast a huge number of really cool special scripted multiplayer levels, including one where you fly around in crate airplanes and shoot rockets and flaming sawblades at each other (TopGun).
There are also some cool mods that died out a long time ago, including "Gutter Runners", a mod based on the Skaven of Warhammer fantasy. There aren't any servers left, which is a real shame... That game had some seriously cool stuff.
Lemme see... There's Jailbreak, which doesn't add any new weapons, but has some interesting gameplay elements. Obsidian Conflict which nobody plays but has some neat levels (Star Wars battlecruiser and fishing), the other co-op mods that I don't know anything about, and then this one mod that I heard about recently called "Ballmen". It has the unusual aspect of being able to walk on walls and ceilings, but looks a bit bland otherwise.
There are also a ton of other mods out there that I haven't tried. I've been out of the loop for a while.
The hitboxes may be off (shoot at the feet! Shoot at the knees! Shoot above the head!), but I like Zombie Master. It's far less competitive than other games, and it has some textbook examples of "losing is fun". Nothing quite like walking directly under the already-falling cargo container, or getting blown into the sky by the exploding trash can.