As part of their constant tweaking of all things App Store, Apple has added a section to the on-device (i.e. on iPhone or iPod touch) App Store that's called "try before you buy".
Here's the deal, on the Featured tab, under New, there are series of 4 banners at the top. If you're in the US, the bottom right one now says Free on App Store. Tap on it and you get a list of New and Noteworthy, Try Before You Buy, and Our Favorites.
Tap on Try Before You Buy and you get a listing of free or lite versions of popular apps you can sort by Featured, Popular, and Release Date.
So, it's not like the Android Marketplace where you can sample any app and get a refund if you hate it; it's really just the same content repackaged in a new way. However, the new packaging will probably be helpful to users looking for just those kind of free, sample apps.
Note: I'm not seeing it in the Canadian App Store yet.