You know that USB drive you're carrying around with you? Toss it. What you may not know about the Palm Pre is that it works as a storage device which you can connect to practically any USB connector that has proper generic storage device drivers.
If you've connected your Palm Pre to your computer (Mac or PC), you've probably noticed three options: Media Sync, Just Charge, or USB Device. If you select USB Device, your Pre acts just like any USB storage device. You can copy files to and from it, and you can even install ringtones and wallpapers as I posted here before.
The amount of storage space your Pre has will be determined by a number of factors: how much total storage space the Pre has, how much the OS takes up, and how much your media files take up. On my 8GB Pre, I have a little over 6GB free. In fact, I use the Pre to write these blog posts, and then post them directly from the phone