One of the things that makes digital distribution so attractive to game publishers is gives them greater control over the value of their products. With no opportunity for a second-hand market, if they say something is $10, guess what, it's $10. I kind of feel like it's cheating at capitalism, but it makes a whole lot of sense, and ultimately it's a practice that just makes me cherish opportunities to get digitally distributed content on the cheap that much more.
Finding Xbox Live Marketplace points cards below retail is the purest way I've done it, but I'm not above game-specific deals as well, although today's deal, brought to my attention by the often-excellent CapcomUnity blog, is more altruistic than most. For today only, you can buy a download code for the Xbox Live Arcade versions of Super Puzzle Fighter II Turbo HD Remix, UNO, and Street Fighter II' Hyper Fighting on Amazon, and receive a second code for the game you purchased to give to a friend. My understanding is that this is a less-than-great version of Hyper Fighting, but I'm happy to fully endorse both Puzzle Fighter and UNO. The HD Remix aspect of Puzzle Fighter's a little half-ass, but as a puzzle game it's still extremely solid. And UNO, well, that's basically one of the best things you can use Xbox Live for.