When Ambient Devices first came out with their Ambient Orb, a wireless softly glowing globe which you can buy for a silly price and pay a monthly fee to have it track something online, I knew I wanted one but not at that price point. Fortunately, many other geeks found this interesting, too, and chose to hack things to make it so. The folks at Ambient even make it easy by supplying DIY notes and sources for parts.
An aside: I used some of those pages to make a Secret Santa gift from a lightbulb. Nifty fun, that. =]
Then I ran across Spöka (pronounced SPOO-kah) at Ikea. Little red, blue, and green ghost nightlights for $13. Soft diffuse light, just like an Orb. Cute shapes with faces. Already got all the power and stuff inside; also turns out to be fairly easy to disassemble, contains a rechargeable battery pack that will run it for 4-6 hours, and has tons of space inside for a builder to tuck in some custom circuits. Have a look at pics from the brief autopsy I did.
I'm pretty sure I can tuck an Arduino board and USB port neatly into one of these; I like the idea of it doing bluetooth and/or wifi, too. That plus some RGB LEDs and this is gonna be way fun. (If you do something fun with one of these, please toss me a comment and a pointer to your project if it's online)
Update: the Ikea website doesn't seem to have Spöka anymore. The Emeryville Ikea has scads of them, though; I may have to go buy a few extra just in case. =]