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Desura has Doorways: Chapter 1 & 2 on sale for $2.49 (75% off) for their Halloween sale.
Comes with a Steam key apparently. According to a comment by the dev on the message board for the game you can retrieve the Steam key from:
http://www.desura.com/games/doorways/keys
Note that you’ll have to buy episodes 3 & 4 later on to get the entire game unless they pull a Valve.