Adrian posted a step-by-step tutorial on how to build an inexpensive logic analyzer for your PC. He even has the schematics and software to download. Having an oscilloscope or simple logic analyzer can save you a lot of time when troubleshooting your electronics project.
A logic analyzer is a like a recorder for digital signals. During a certain (small) period of time, the state of a few digital lines can be recorded to a file. An event can be specified to signal the start of the recording, i.e. line 1 toggling from 0 to 1.