<link=http://www.gog.com/en/gamecard/shogo_mobile_armor_division>Shogo:Mobile Armor Division. You've either played everything else I would recommend, or it's already been mentioned, but Shogo is great. It came out slightly before Half Life, did a lot of the things that Half Life became famous for first, but got pushed aside by gaming history because everyone was so hyped for Half Life at the time that very few people bought it. Note that it didn't do everything that Half Life did, though; it definitely doesn't feel like a clone, either of Half Life or anything else. The multiplayer was some of the best and most frantic I've ever played, but the Devs spent most of their time on singleplayer, and it shows. It's from the same people who made F.E.A.R., but it's decidedly more old school than that game; give it a shot, you should like it.