Input devices. Joysticks, Gamepads, Dance Mats. Of course also keyboards and mice which (in combination with TV-Out and Easy Debian) could make for a ultraultraportable Desktop experience. Another good use is when you need to collect data from different people (i.e. at the end of a holiday, you want to copy all photos directly via USB from the cameras / a card reader). I'd also like to find an ambient display kind of thing (something like the Ambient Orb) that can then be used (with some scripting on the N900 side) to subtly notify me about incoming mail, a broken service or something else that needs my attention.

Or use an USB sound card to give your N900 a line in for recording audio directly from an analog source.

There's a wide variety of USB crap gadgets out there for you to choose from