I built an interface to the parallel port for use as a logic probe. I found this program that, using its own port driver, can sample the port at up to 1MHz, on 8 channels.
I built a pretty interface into a box with switches for the ground bus, plus I made some 2 conductor probes and some singlle conductor probes. Unfortunatly I couldn’t find any of those handy circuit clips, so I just have lame aligator clips on the probes.
While looking for something to actually do with the probe, I built a simple square wave oscillator with an LM555 timer. It was almost cool… and everything is cooler when it goes faster. So I started messing with the circuit to make it go as fast as possible. I don’t have an very small capacitors, but with what I had I got the thing to 675.68 kHz. I think that’s pretty good. I poked around and got it to go a little faster, but at that point the probe began aliasing because it couldn’t sample any faster, so I couldn’t test how fast it was going.
I’m going to try to use this to reverse engeneer chips, ie monitor the memory bus and see what a particular chip is doing, but with only 8 channels and up to 1 mHz (usually about 800 kHz) it’s unlikely that I’ll find something simple enough to use it on. But hey, maybe I can still get linux on my toaster 
I’ll keep a log of things I do with the probe here.