Our friends over at Wired posted this awesome beginner electronics project that mashes up a cheap digital camera and an Arduino board to create a "camera trap" -- a camera that snaps a digital picture when triggered by an external switch, in this case an IR emitter/sensor loop. Whenever something breaks the beam, you get a pic. It looks like a pretty straightforward project and it shouldn't be too pricey.
And on that note, I've been tempted by the Arduio intro projects (available from several sources) recently. I love the idea of a "homebrew" style board that's actually programmable in a language that won't take me years to decipher. I think I'm going to add one to my Christmas list.
Speaking of Christmas, here's an awesome application of this type of device: spying on your favorite tree-climbing cat:
Check the Wired How-To Wiki for lots of details including a materials list and the Arduino code you'll need to make the whole thing go.
Build a Camera Trap [Wired How-To Wiki]