You might recall the "Ad Hoc" distribution model, which was used by the developer of the "booted" Podcaster app to distribute his program. That really got him into hot water; its actual use was for beta testing or promo copies. It was a pain to use for promos,though, as developers had to custom build the app for each user. But finally, Apple has given developers a way to hand out promo codes.
Developers can hand out up to 50 promo codes for each version of an application. As that implies, the counter gets reset when a new version is uploaded to the App Store. A user simply types a code into the "Redeem" section when checking out.
One more caveat besides the 50 code limit: it's only available in the U.S., for now.
While this is cool, it would be nice if Apple allowed ... heaven forbid ... trial versions of applications. It's something that developers and end users have been clamoring for since the App Store opened.