Amazon's weekly deal on downloadable PC games for the week of January 22, 2012 includes a dash through Sonic the Hedgehog's past games with Sonic Generations and a strategy game set in feudal Japan with Total War: Shogun 2.
Get Sonic Generations or Total War: Shogun 2 for $14.99 from Amazon until January 28. If you purchased a downloadable PC game from the online retailer during its holiday promotion in December, you can use the $5 coupon you received on these titles.
UPDATE: Amazon fixed the price for Total War: Shogun 2 and it is now at $14.99
Unfortunately, it does not look like Sonic Generations activates on Steam right now but Total War: Shogun 2 does activate.
Daily deals for downloadable PC games start on Tuesday and include titles like Battlefield: Bad Company 2 - Vietnam, Darkspore, Need for Speed: The Run and Battlefield 3.