I recently got a Blackdog device to try some ideas out. The Blackdog is a very small Linux server with no screen or keyboard.
You plug it into a PC via USB and it emulates a USB CD device. The host machine then autorun's this CD. If the host is a Windows machine it starts an X client. If it's a linux machine it does nothing.
Once that is done the Blackdog then stops pretending to be a CD and pretends to be a USB ethernet device and sets a private network between the host and the Blackdog. The running X client on the host machine then connects over that network to the X server on the Blackdog. Through this you can run programs like xterm and have them display on the host. The Blackdog also does fingerprint authentication during this process to ensure the user is authenticated.
It's a pretty neat little device and has some interesting possibilities. It works flawlessly connecting to Windows XP machines. For Linux machines there are a few issues as newer kernels need a patch applied to it. But with a little tweaking I've got it working on a number of machines.
The blackdog has a PPC based processor. To develop for it you use the Blackdog SDK. This includes a QEMU based system which emulates the Blackdog. You compile the software from in that and then scp it to the Blackdog. Alternatively you could apt-get the development tools on the Blackdog itself.
I've successfully got a number of programs running on it including Factor, Erlang and Yaws. These run very nicely in fact. Hopefully I can develop some of my ideas so it becomes more than an interesting gadget and something actually useful to me.