I haven't actually done any development but I would like to and a perfect first project would be using small computers on my sailboat. This page is really just a scratchpad at this point.
Hardware
arduino - Seems to have a huge following. Step up from a PIC.
gateworks - Found them via dd-wrt.
gumstix (COM) - Detailed community wiki with a lot of linux info.
zigbee - Ad-hoc wireless protocol (not Wifi). Not sure if this is open source enough yet. Needs more research.
raspberry pi - very inexpensive
panda board - touted as a mobile device dev platform (more powerful and meant for driving peripherals on mobile devices)
technologic systems