- SSH to serial converter. Plug a couple of USB-RS232 cables into the Pi, plug them into serial consoles on servers/industrial equipment/whatever, and you no longer have to crouch behind a rack with a laptop whenever you need to monitor/control stuff.
- Print server for a USB laser printer
- NAS/backup machine/torrent downloader/etc
- New guts for a broken NES.
I'm sure the closed-source GPU and ARM11 CPU offend some people, but hey, it gets the job done in places I've stuffed it.