I find these a particularly good deal, but for my Australian friends, steam deals are a godsend.
Steam generally has a "weekend deal" where the price of a game is drastically dropped. These deals are limited time only, so I try to get the word out as quickly as possible.
In the past, I've picked up the id pack for 25% off, The orange box for 10% off, the epic pack for 10% off, and most recently, Left 4 dead for 50% off.
Sometimes deals pop up mid-week, such as when I picked up Gmod for $5.
This week, THQ is releasing a different deal each day. Today, Saints Row 2 is 75% off
So, $7 dollars about. I don't think my system is quite up to playing it, but it is a deal not to be missed.
It really does want to download 11 gb of assets. That's a lot of stuff.