Since I see that a lot of ppl don't know about this I guess and a few people are using that to their advantage and then other people accuse these people of that and so on and so on.
It creates a huge mess in my opinion.
So on the "INTERNET" there are often chances to buy great games very very cheap like a 1$ cheap.
Now you might say what kind of sorcery is that mr_ice and I will respond the sorcery of goodwill and tiny bit of close bankruptcy.
So these games are usually sold in bundles and there are a few very famous sites that you can buy them off that have these sales from time to time. Games vary greatly from AAA titles to great unknown indie titles.
What's nice is that almost all of these sites have a newsletter so you can put your email and receive news when new bundles start.
List of sites:
Humble Deep Silver Bundle (pay what you want and help charity)
Humble Weekly Sale: Introversion (pay what you want and help charity)
All Game Bundles
Active Bundles | Indie Game Bundles
Awesome Indie Games for Free - TheFreeBundle
The Starry Nights Bundle - Indie Royale
The Indie Gala
Bundle In A Box - Capsule Computers Bundle
If you know more feel free to post and I'll add them.
Also all of these pretty much support steam which means you get a steam key.
For the rest of them (not steam supported) you can go here and read it Bundles list
Have fun and shop smart