I highly recommend keeping your eye on Steam.
As of late, they have a lot of sales for a lot of games.
For example, you can pick up CS:Source for $6:80 for the next 24 hours.
Another example is GTA IV, which is going for $14.99 right now. GTA IV is pretty god damn awesome, so that price is practically giving it away.
Anyway, check out the Steam store and the sales.
My reference number is: r7gg9h