If you think Steam is good, try Greenmangaming.. MUCH cheaper then Steam for virtually anything, and they always allow coupon stacking.
For example they had Dishonored for $19 today, and sold out of keys, while Steam has it for $29.99 right now. I picked up Shogun2 + all DLC for $5.00 from them. Insane deals.. I rarely buy off Steam anymore because of GMG.
For example right now they have a 25% off coupon code that STACKS with the other discounts. So you guessed it, a 75% off game, you can apply another 25% off code.. That current code is;
GMG25-G4VDR-0ZL4Q
Trine will cost you 98CENTS right now. I can't even begin to tell you the deals I have gotten off of them.. I bought the preorder Sim City Limited Edition off them for $27 last week, and that's $60+ everywhere due to it not even being out yet. Insane.