I have been thinking about buying a nice Oscilloscope/Logic Analyzer ever since I bought that little cheapie one I bought a few weeks ago. But then I thought that it would be cool to build one. I know how they work, and usually my needs are pretty low tech and low bandwidth. As a result I am going to try to build one using either an Arduino or Netduino.
Here are some of the plans I am reviewing, mainly since it will definitely be more expensive to build then to buy, but it would be cool to have the experience:
http://www.practicalarduino.com/projects/scope-logic-analyzer , this one looks good!
http://www.instructables.com/id/Girino-Fast-Arduino-Oscilloscope/, fast, but complicated, if you have built out other kits or projects, this is for you.
http://mitchtech.net/arduino-oscilloscope/ cheap, requires only an UNO board and a wire
http://sebastiannilsson.com/en/k/projekt/realtime-plotter/, cheap, not a true Oscilloscope, but close enough and simple.
I am definitely going to give one of the harder ones a try, but the last two are likely good enough to solve most problems that require an O-scope (Oscilloscope).