The particles.js library really only does one thing, but it does that one thing well. If you want to create a really awesome looking demo involving randomly roaming particles — with or without connecting lines — this is absolutely the library for you.
Roughly 60 lines of JavasScript to create 200 beautiful particles. The result is something more than a little hypnotic. I love restarting it over and over again to see what different colors I end up with.