Downloading past purchases from the App Store, iBookstore, and iTunes Store - http://support.apple.com/kb/ht2519 - enabled with iTunes 10.3 and newer; not available in all countries; apps, books (not audiobooks), music, t.v. shows, and movies (some - not all studios have permitted this). Movies currently available in the USA only. Downloading previously purchased movies and TV shows requires iTunes 10.6 or later. Discontinued items not available. For items not included in the iCloud list (e.g., ringtones), or locations or computer systems where iCloud is not (yet?) available, you only get one download per fee paid. Apple notes it is your responsibility to back up your purchases.
Select the store on the left side of iTunes.
Click on Purchased on the right side under Quick Links.
You can re-download your available previous purchases.