A quick note on any of the EA games that you can purchase on Steam--you can take the CD Key and enter it into Origin and get the game unlocked for you on their service as well.
For example, if you purchase Mass Effect 1 and 2, you can take those CD Keys, enter them into Origin, and then you can pick up ME3 for something like $7.50 on their service to complete the trilogy if you're so inclined.
And no, I'm not saying Origin is great--in fact, it sucks pretty hard as an online service goes, but if you're a fan of EA games on PC already (and therefore are already subjected to the inconvenience that Origin is), then you may as well put that CD Key in and have that game unlocked on your Origin account as well.
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Also a few points:
-Steam had Saint's Row IV for $19.99 and the DLC pass for $9.99, which is a great deal. Great silly GTA-style sandbox shooter that doesn't take itself seriously at all, and has good co-op features to boot.
-If you go with StarBound, please note that the game is in beta and your characters are subject to being wiped at any time until the game is in General Release. If this is a problem, go with Terreria (only $2.49 via Steam Sale) until StarBound is out of Beta. And for the record, both are awesome, addicting games.
-For the next three hours, Borderlands 2 is $7.49 by itself, and $29.99 for the GotY edition (read: with all available DLC). This game is an awesome co-op shooter that works perfectly and seamlessly with either a KB/M or a 360 Controller and an example of what PC games, as they creep to the living room, will be like in the years to come.
-Stu, welcome to Steam. It's an excellent platform for gaming, and once the 4GB graphics cards come down in price later this year and Steam OS/Boxes are widely available, it will easily be an affordable living room option, not unlike the XBox One, Wii U, or PS4.