Mate, a little more info on what it is that you like, or want will go a long way to helping us help you. "Shooter" covers a whole lot of ground, and there's really not going to be a proper answer for you. However, give us some extra info, such as your system limitations, weather you want internet gaming, good story, futuristic, realistic? There are all sorts of different styles to suit all sorts of different tastes.
For me, I'm not a fan of any of the games mentioned, as they seem to be too fast and "fake" for me. I prefer realism type games. But these are generally VERY hard, and very tedious, with long breaks of little action. It doesn't suit everybody. Unreal Tournament 3 is about as opposite as you could get, and appeals to many because it's fast and completely furious.
Then we can get into mods!

Mods are "modifications" of existing games, and are almost always free, so long as you have the original game they're based off. A game like Battlefield 1942 was a WWII shooter, and the mod Desert Combat brought it to present day, set in Iraq during the first Gulf War. It changed the game so widely it's regarded as it's own title - but still needs BF:1942 to run.
So there are SO many "flavours" out there that it's really hard to suggest a "nice shooter". Give us your basic system specs, your budget, your preferences and likes, and maybe we'll be able to give you something better to aim for.