I'll second Half-Life 2, along with HL2:Episodes 1 & 2. Borderlands is awesome as well, definitely worth picking up the GOTY edition on Steam when it's on sale.
HL2 is a significant game in game dev history and technology sense, but I'm honestly not very impressed by the game either action- or plot-wise. Its strengths are more in wandering around nice scenery, and solving the tacked-on physics puzzles is fun too. The main problem with the action is weapons feel way too weak. The feel of the SMG and the (lack of) reaction of human enemies getting shot is especially terrible, despite the gun being the most common in the game. Funnily enough this was a problem with first HL's human enemies as well, but was easier to ignore because HL didn't have many of them, whereas HL2 is packed to the gills with human enemies.Absolutely. Seriously, you are not a "real" gamer if you have never played HL2...even if it's old already. And Borderlands is amongst the best (if not THE best) for co-op ..it's just awesome fun with some roommates/GF/Wife etc. on the LAN
Borderlands was a lot of fun and is cheap now on Steam -- get the full package with all of the DLC.
It's often called "Diablo with guns" because it's a shooter with tens of thousands of different guns as loot drops, with similar trash, uncommon, rare loot drops.
HL2 is a significant game in game dev history and technology sense, but I'm honestly not very impressed by the game either action- or plot-wise. Its strengths are more in wandering around nice scenery, and solving the tacked-on physics puzzles is fun too. The main problem with the action is weapons feel way too weak. The feel of the SMG and the (lack of) reaction of human enemies getting shot is especially terrible, despite the gun being the most common in the game. Funnily enough this was a problem with first HL's human enemies as well, but was easier to ignore because HL didn't have many of them, whereas HL2 is packed to the gills with human enemies.
Long story short, the game is surely worth playing - once - but I recommend playing it on Normal difficulty or lower so that the enemies hopefully feel less like they are made of Teflon. The follow-up Episodes are pretty much the same ol' but so short that you might as well play them too.
This. It's free and has an old-school feel to it.