IShowU may not always be free, but while it is, here is a free app for creating screencasts. Screencasts are recordings of the computer screen, often with an audio track narrating what's going on. With a screencast, you can teach your students how to perform certain tasks on the computer, or show your future customers how your program works. Show them instead of tell them.
iShowU is brand new and may still have some quirks, but creators Shiny White Box are working hard on it and it looks very promising.
Update: This application is no longer freeware. Bummer. Another one bites the dust.