• Tweeting kegerator with temperature logging and status updates
• "Cornhole" bean bag toss game with scoring sensors and electroluminescent wire lighting

Making Things Talk
by Tom Igoe

A wide-ranging introduction to making interactive objects using Arduino. Has some great whimsical projects and is good for sparking your own ideas.

Arduino Cookbook
by Michael Margolis

A reference containing solutions to many commonly encountered programming tasks. Starts from the basics (e.g. how to declare variables, evaluate statements, and call functions) but quickly moves on to more complicated and specific tasks (e.g. how to post to Twitter, wire and trigger relays, and use the Time library.)

Practical Electronics for Inventors
by Paul Scherz and Simon Monk

Which kind of capacitor should I use for this? What's a Darlington transistor again? How do I make an emitter-follower? A reference guide to electronic components with a bit of theory and a lot of practical advice. Almost all of this stuff is out there on the internet, but it is nice to have it all in one place, written in a consistent style.