I don't remember exactly where I first heard of this, but it's been around online for a very long time.
If you're on a mac or linux system, open the terminal and enter "telnet". When the telnet terminal opens, enter "o" (as in open), then enter towel.blinkenlights.nl.
Neato ASCII entertainment ensues.
It just makes me so happy that someone put the time into making this, and that it's still online after all of these years.