Ask Slashdot: What's The Best Open Source Hardware to Tinker With?

This question comes from an anonymous Slashdot reader who just got an Arduino and started tinkering with electronics: I'm quite amazed at the quality of the hardware, software, and the available tutorials and (mostly free) literature. A very exciting and inexpensive way to get a basic understanding of electronics and the art of microcontroller programming.

Now that I'm infected with the idea of Open Source hardware, I'm wondering if the Slashdot community could suggest a few more things to get for a beginner in electronics with experience in programming and a basic understanding of machine learning methods. I was looking at the OpenBCI project [Open Brain Computer Interface], which seems like an interesting piece of hardware, but because of the steep price tag and the lack of reviews or blog posts on the internet, I decided to look for something else.
Leave your best answers in the comments. What's the best open source hardware to tinker with?