If you are looking for single player action you would need to ask Dan. I don't play any single player FPS. However I would recommend getting Demos for FEAR
and Quake 4
. You should be able to get Far Cry
cheap now and should enjoy it. The Call of Duty
Series is also decent. If you liked Half Life 2 then you would probably like FEAR and Far Cry the best. FEAR though is massively taxing on systems.
For Multiplayer I recommend the Battlefield Series, including Battlefield 2
For no cost I highly recommend some of the free Shooters here: XP Games Guide (Action)Highly Recommended:America's Army
(An excellent realistic tactical shooter that is constantly maintained and updated with millions of registered users)Wolfenstein Enemy Territory
(The free multiplayer version of the retail game)Recommended:Nexiuz
(Very high quality if you like death match)Sauerbraten (Cube 2)
(Still in developement)Tremulous
(Team based Multiplayer with more depth and classes such as engineers)Alien ArenaNot Bad:Legends
(A clone of Tribes but with better graphics)Tribes
(good but dated and the amount of players on servers is limited)
The great thing about the free ones is it costs you nothing to play them. I really need a new computer to play the latest games so I haven't invested any money in them.