Global Operations was mentioned by Arris, and man is it a sweet game! However, you mention that you play CS a lot, and this game is like CS in many ways (also improved in many ways). So if you are looking for a new version of CS dowload the mulitplayer demo of Global Ops.
Also, have you ever played Serious Sam or No One Lives Forever? NOLF can probably found for cheap now, and it is a GREAT, underrated game. Serious Sam:Second Edition just came out, and it is supposed to offer even more action than the original, with hundreds of baddies on the screen at once, and even better graphics.