Not a huge discount, but it's best I've come across.
http://www.greenmangaming.com/games/action/duke-nukem-forever/
You purchase a key that you activate on Steam.
edit: I think it's a weekend only sale
http://www.greenmangaming.com/games/action/duke-nukem-forever/
You purchase a key that you activate on Steam.
edit: I think it's a weekend only sale