There are lots of people out there working with this stuff which is very exciting.

If you know of other good sites please comment and we will add them in.

The official Arduino site - http://www.arduino.cc/

The World Famous Index of Arduino Knowledge - http://www.freeduino.org/

Simple freeduino kits and much more - http://moderndevice.com/

the make blog arduino tag - http://www.makezine.com/blog/archive/arduino/

cool blogs by people working with arduino

wearable arduino clones - http://www.cs.colorado.edu/~buechley/projects/projects.html

Near Future Labs (Tod Kurt) - http://www.nearfuturelaboratory.com/ and http://todbot.com/blog/

uCHobby - http://www.uchobby.com/

ladyada (excellent tutorials) - http://www.ladyada.net/learn/arduino/index.html