This is the current version of my Arduino-powered, low-resolution, multi-channel oscilliscope. I made 2 frontends, that both speak firmata, so you will need to install the Firmata firmware on your Arduino (File -> Open -> Examples > Library-Firmata > StandardFirmata, in latest Arduino IDE.)
It used to run over a simple serial protocol, but I have decided to standardize on Firmata, so I can keep the pin-reading code in the client, and make installation more standardized.
The basic idea with both front-ends is to connect your Arduino running Firmata, and start the front-end.
If you don't care about developing the front-end, or don't want to install any dependencies, or do anything other than just have a working oscilliscope, choose the correct "standalone" zip file for your plaform from here.
This is a up-to-date port of my original oscilliscope. It is also the current source for the stand-alone apps.
If you want to modify the processing front-end, see instructions.
This is by no means the most efficient or best osciloscope you can build. It's meant to be simple & easy to modify, so you can do whatever you want with it. Checkout this wiki article to see how to easily read a different voltage range, for example.