Stratum 1 NTP time server: http://rdlazaro.info/compu-Raspberry_Pi-RPi-stratum0.html
D-Star Acces point: http://www.k6jm.com/hs-piconfigG4KLX.htm
Weather Station: http://www.weewx.com
(just started working on this one).
It's nice just having cheap Linux boxes around for experimenting and trying different things. The original point was to allow programming students to mess up a computer that they didn't need for the rest of their classes. Since it just uses SD cards it's a fairly simple process to start over. Even though it will work with 1 Amp power supplies like cell phone chargers, I always connect with 2 Amp supplies, usually used with tablets. My DV access point runs with a high capacity external battery so I can use it mobile without having to worry about losing power (can cause data to be corrupted if not powered down properly).
One other thing I'd like to do is experiment with remote rig control and audio, using the Hamlib libraries and JACK audio. I've messed around with the hamlib libraries running as a daemon on a laptop on a LAN and it worked very well. FYI, hamlib is a standard interface for ham radio devices (radios, rotators, etc) that can run in the background. JACK is an advanced low-latency audio routing application that can run over networks and other devices. It might be interesting to see if there's a way to integrate an iambic key, perhaps using MIDI or some other method too.