Mac only: iPhone users have been able to browse the contents of their device using apps like previously mentioned iPhone Explorer for awhile, but Phone Disk integrates it with OS X, mounting your iPhone in the Finder instead of a separate app.
Whether you need to get into your iPhone's file system to extract music and photos, or you just want to take advantage of some of the unused space on your phone like a USB drive, Phone Disk will make it a seamless experience. Phone Disk sits in your menu bar, waiting for you to attach an iPhone, iPod touch or iPad to your computer, automatically mounting it when you do. You can view the file system as you would any other drive in the Finder, and just as you could with older iPods in disk mode. Best of all, it works with any generation of device, and there is no jailbreaking required.
Phone Disk is a free download, Mac OS X 10.5 and later. The app is only free until September, so download and activate your copy now while you can. You can find the activation code on the download page.