This is a good solution if you just need to record something, but don't have a microphone. I don't believe this works with every computer - audio port - headphone combo. I think that it doesn't work on computers that have that "other" microphone port (a little deeper, I think?). There are two sizes, I don't know what the terms for those are.
[robg adds: This didn't work on either of my machines, a 12" PowerBook and a first generation Dual G5.]

