Most projects like this are called "inexpensive oscilloscopes", however that's a bit of a stretch. Instead we like to think of them as methods of displaying waveforms that can be measured with an Arduino - and the following example by Instructables user "Superbender" is quite a good one. Using a typical Arduino Uno-style board you can display the values from between one and three channels relatively simultaneously using a simple processing program on a PC. The waveform updates are inversely proportional to the number of channels being measured, but if you're in a hurry and don't have an oscilloscope this is basically a free substitute.
For more information and the required code, visit the instructable. And for more, we're on twitter and Google+, so follow us for news and product updates as well.
If you're new to Arduino, the first step is a solid board for your projects - our Freetronics Eleven - the Arduino-Uno compatible with low-profile USB socket, onboard prototyping space and easy to view LEDs:
