If you have Half Life 2, you can get the Synergy mod and play it multiplayer. A friend of mine played through HL2 and EP2 in it and we have a few good memories still. At the beginning of Episode 2, I picked up a rock, hurled it in the air, and positioned myself under it. My friend never saw me throw the rock, so he just saw a rock fall out of the sky and instantly kill me in an explosion of gore. Good times...
You could also try watching the steam sales each day, like I do. When something good looking comes up cheap, I try it out. Scroll down a ways on the main store page and click the specials tab on the game list for a full listing.
