Windows/Mac: Unfortunately the iPhone and iPod touch lack the ability older iPods had to run in Disk Mode. That's where iPhone Explorer steps in, allowing you manually manage files on your device, or just use it as a flash drive, without jailbreaking.
iPhone browser is a lightweight application that has an easy drag-and-drop interface that lets you browse the contents on your iPhone. You can manually transfer your photos, music or movies from your device back to your computer, or just add files to the iPhone and fill up the unused space as extra storage. There's even an image preview for transferring photos. The best part about the whole thing, though, is that you don't need to jailbreak to use it. If you do have a jailbroken device, though, iPhone Explorer becomes even more useful — It can access the root of jailbroken phones and browse the files inside, with no need for complicated SSH setups.
The only issue I had was that multiple selection didn't work so well — whether I was trying to transfer files to or from my iPod touch, doing it one file at a time worked much better than trying to do more. So if you need to transfer large batches of files, you may be better off putting them in a folder beforehand to avoid crashes.
iPhone Explorer is a free download for Windows and Mac OS X 1.5+, requires iTunes 8 or 9 to use.