Plug computing has been around for about a year, but it is still a little tricky to get your hands on the hardware. TonidoPlugs are back ordered a month at the moment. Amazon has PogoPlugs, but they apparently have less memory, and I'm not sure how readily hackable they are. PlugComputer.org links to vendors for reference hardware and development packages. Note that the GuruPlug is generally regarded as a better compact space heater than computer (my TonidoPlug is very cool).
Oh, price? $99, more or less.
See also PlugWiki.