I just bought the tiny and inexpensive Arduino Diecimila microcontroller board from the O'Reilly Maker Store. All information for the hardware is available on the manufacturer's site. And, the IDE (Integrated Development Environment) used for programming the board is Open Source...

Arduino Software

IDEs are available for Linux, Mac OS X, and Microsoft Windows. I'm looking forward to using it to build a few projects this spring and summer. If anything comes of this plan, you'll see longer form blog-articles over on the O'Reilly Mac Center.