So I found two 3V devices that comunicate over some sort of serial. (maybe its one wire) I have the two devices talking to eachother, but I want to hack them with my arduino. Can someone tell/show me how to make a sniffer to listen in on the serial connection? (transfer speed is not a problem, the devices are very slow.)
Use an arduino oscilloscope.
Alternativly you can use the sort of thing people use for sniffing IR remote controllers.
Depending on whether the waveform is periodical, simple oscilloscope might not work because the trigger point will shift and the waveform won't be stable.
If you have a digital scope, you can use the single shot function which will be able to capture arbitrary waveform. Of course, you can always use a dedicated logical analyzer.
you can also use a sound card to record it to a file and analyze the waveform.