hl2:dm \o/ (you probably already have it...) throwing toilets at people never gets old
other than that, single player probably Dark Messiah, mainly for the really smooth combat system, and
multiplayer I finally bought Red Orchestra the other day. Have been meaning to since the first free weekend. When you get it right it's one of the most satisfying games you'll play, picking someone off from 100 metres with just an ironsight, or using the range adjustment in a tank to hit exactly the spot you were aiming for. Was playing last night exchanging bullets with someone on Odessa, I was in the tower with a rifle and he was on the roof opposite with an smg. He shot the gun out of my hands about 6 times before he finally managed to hit me.
Not the kind of game you should buy without trying first though.