Warhammer 40K: Space Marine $12.50 - registers on Steam
I've been waiting for a good deal on this one and it has arrived. Just saw this over at slick deals. Cheapest price yet.
You can get it a greenmangaming.com. Site is reputable (only heard good things over on slick deals). You'll get the serial and can then register that on Steam and download direct from Steam.
Here's a link to the deal: http://slickdeals.net/?sdtid=3786944&sdfpid=66738&sdop=1&sdfid=9&u2=http://www.greenmangaming.com/games/action/warhammer-40000-space-marine/
Here's a link to the thread I found the deal in: http://slickdeals.net/f/3786944-Warhammer-40000-Space-Marine-PC-Digital-Download-Registers-on-Steam-12-50
If this is one you are interested in, this is an excellent time to pick it up.