You can read iTunes 10 .5 release note:
The ‘Purchased’ screen is a really handy feature to easily and quickly find a previous purchase and instantly download it or even download all of your previous purchases with just one click. The feature, which is also included in iOS 5, is part of Apple’s iCloud, which was unveiled at yesterday’s WWDC keynote. The ‘Purchased’ screen also gives you some fairly powerful tools to find the exact purchase you want, either from just searching, only displaying items not downloaded to your iTunes library or just list the purchases alphabetically.
You can download iTunes 10.5 from Apple’s website, or directly on your Mac through Software Update.