I think it would be nice to have a logic probe and mini scope project. Maybe have it in the playground. Not all of us have access to these and sometimes buying a commercial solution is really not needed. It could probably use Processing for the software and would thus be cross platform. And it'd probably be easy enough for people to build as a breadboard arduino or as a shield.
What do you guys think?
a logic probe would be pretty easy... just use a bunch of if/then conditionals to test for things like pulse trains, serial sequences or just plain high/low levels, and then set an LED if the condition is true. Why don't you build it and let us all know how it's done? 
A scope would take a few LED Matrices, and driver chips, go up to a few MHZ in speed and show basicaly only square wave pulses. It would also take a long time to assemble. Compare that to a used 20Mhz scope that can usually be had for $100. In the end the used scope will always function better, and you get the added benefit that you're recycling.
The scope has been done before PIC18F2550 KS0108 Graphical LCD Oscilloscope. The chip looks similar (in specs) to a Mega32, but a lot of the code is for the LCD. He also has a version that displays in software on a host machine and it's a lot smaller. (He actually made his version as an HID device hehe) But that's just one example, I've seen others, even CRT PIC based scopes. I will look and see what I can do for a logic probe, but the scope is beyond my skills 
But it can be done for very cheap. Bringing the display to the computer makes things a lot easier. From the little I dabbled with Processing it also looks like it's very well suited for graphics so that shouldn't be too bad. I actually think it would be quiet quick to assemble. It might not give Mhz range, but 0Hz to ~600Khz is still useful. Specially with the frequency generator (there's some chatter in a thread by CosineK.) which is in that range. Also, afaik using (fast) fournier transform it's possible to obtain full sinusoidal waves.
I know there are many programs out there for PC based scopes that require you to buy like a $200-$300 probe interface that sends data via USB or serial port. Mabey you could just find out what kind of input those pre-existing programs require, and then just use the Arduino to supply that interface. Just a thought- easier than having to write an entire scope program yourself.