GPIB to USB Adapter
Ever since I got my HP 1630D logic analyser, I've been wanting a way to dump acquisitions to my PC. The analyser only has a GPIB port. Adapters usually go for well over £100 on eBay, so there's no chance of me doing that. My TDS420A oscilloscope has RS232, but the functionality is limited to screen captures. With GPIB it's possible to dump acquisitions, and configure almost every aspect of the scope from the computer.
I came across this page which provides code and details for building one with an Arduino. It's very simple to build - I ended up cutting up a male Centronics connector so that it can fit into the 24 pin GPIB port. Anyway, after getting it wrong a couple of times (I'm bad at geometry), I had a fully working GPIB adapter. Pretty nice!