First up, Call of Duty: World at War Zombies [iTunes link], which brought the undead to the WWII shooter, has a new lite version to play for free, which plays exactly like the $9.99 full version [iTunes link], but for only three rounds. Strangely enough, it's actually the exact same size download.
Rock Band made a splash when it debuted on the iPhone late last year [iTunes link] at $9.99. Since then, it's dropped to $6.99, but there is now a free demo available [iTunes link] that features only the Foo Fighters' "Learn to Fly." If you want to check out the controls, though, and perhaps see how they compare to the console versions, it's worth a download. We kind of like this precedent of releasing best-sellers for free after a delay, though there's probably still some tuning to do in just how much content is enabled in these trial versions.