Over on YouTube user sn0ren has posted a well produced introductory video to RTL-SDR. If you're already familiar with software defined radios and RTL-SDR, this video won't be of any use to you, but if you are just starting out, or have friends that might be interested, this is a great video to show.

Sn0ren's video goes over basic concepts like the history behind RTL-SDR, what an SDR is, what radio is, and then goes on to explain and demonstrate reception of the various different types of  signals that can be received with an RTL-SDR and compatible software.