How good is your computer? Mine is crappy so any games I suggest will be at least 5 years old. If not more. Probably from the 90's.
Heroes of Might & Magic: Dark Messiah, if you have a fancy computer. FarCry, a good FPS, with both leaning around corners, silenced weapons, and the ability to go prone, which all FPS should have, the original Half-Life and expansions, if you liked the Orange Box. It's like Half-Life 2, but with more strategy in fight than the run-and-gun of HL:2, and more weapons, while your playing Half-Life might as well play System Shock 2, Fallout 1 & 2, good old school RPG's that are very RPG-y and it can have lots of cut's and dying animations, if you like gore as well as shinies, Death To Spies, I haven't played it but it looks cool and it has lots of Sniper Rifles, Morrowind, really awesome and so much shiny stuff that they become mundane, Brothers in Arms: Road to Hill 30, a good tactical WWII game without the annoyances of the ones set in modern times, Stalker: Shadow of Chernobyl, it's d fun with a cool environment and different, Dungeon Keeper: 2, a very un-RTS/god game like RTS/god game, played from the evil side, Hitman series, fun and have sniper rifles, and Overlord, it has shinnies and good 3rd-person-evil-action-hyphenated-medieval-game-play.
I tried not to broaden your horizons too much. I don't want to blow your mind.