Screeny is a Mac app for capturing what you're doing on your screen and sharing it with others. I use it for showing clients how to use the various applications I recommend to them for managing their website, but it's also great for when you're having an issue on your computer and want to show a friend who's helping you troubleshoot the problem. Just capture a video of it and send it off so they can see exactly what's going on.
Here's a screen capture of how Screeny works, including cheesy stock background muzak:
Free is free. Grab a copy even if you don't think you need it.