Sent to you by Chris Hunter via Google Reader:
Windows/Mac: Your iOS device has a limited amount of storage. It can store a lot of junk you don't need over time like temp files, scripts, and failed sync files. PhoneClean is an app that deletes all that junk for you.
All you need to do is launch PhoneClean, attach your iOS device to your computer, and click the "Start Scan" button. PhoneClean will dig through your device for a few minutes, and will give you a report on what it can safely delete. Check the boxes for the junk you want to delete, and PhoneClean does it. I didn't reclaim much space myself, but your mileage may vary.