Steam is a great service. I've bought over 200+ games with it and completely stopped buying games from brick and mortar stores. A lot of them are from sales, and now they're introducing one new sale every day too.
Speaking of which, just last night I spotted a really fun game on Steam - Terraria
. I love games where you can build, dig and explore. If you have to compare, it kind of reminds of Minecraft, but is still different and has a lot more features. The multiplayer works great too and you can have teams. It's great fun when both teams build a castle and start attacking the other one, even making secret tunnels to get there unnoticed, and there's tons of weapons and other fun stuff in the game. YouTube has a cool video series on the game