For the electronics enthusiast, an oscilloscope - which turns readings over time into a handy waveform graph, allowing you to see exactly what's happening with a given circuit or component in ways that a multimeter cannot - is a great tool, but one very few can justify. Even the cheapest modern bench-top digital oscilloscope can cost three figures, putting them out of reach of many hobbyists. One solution is to join your local hackspace, where an oscilloscope is bound to be present; another option is to build your own, as Martin Vaupell has demonstrated over on business-to-business electronics supplier Element 14's blog. Using both an Arduino Mega and cheaper Arduino Uno, to prove it would work with both, Martin worked through a series of examples found online before stumbling on one that works with the latest Arduino IDE: Noriaki Mitsunaga's Japanese-language guide. Coupled with a library for his colour LCD screen shield, Martin was able to build a two-channel scope quickly and easily - and while its performance can't rival that of a commercial equivalent, the homebrew version is considerably cheaper.
What I'm most happy with is the two channel feature. I often use the second channel as a reference point to be able to differentiate the main channel on the screen.