An 1GHz active differential probe project by Daniel Kramnik:
This project was an ultra high input impedance (0.5pF||30MΩ or better) high bandwidth differential oscilloscope probe designed, built, and revised over the course of two weekends for the 2014 MakeMIT hardware hackathon. It can be built for about $30 in parts, plus the cost of etching or sending out a PCB and (optionally) 3D printing a chassis. A list of downloads necessary to build a complete probe is available at the bottom of this page. The project was in the top 10 the first week, advancing the team to the second week, where it took second place.
The objective of the project was to provide hobbyists with an affordable means to make high speed measurements, in which respect I would call it a great success. I was able to use the probe to measure various data converter waveforms from my digital oscilloscope project that I had otherwise been unable to get good data on.