arduino.cc  – tutorial, examples and lots of help on the forum!

http://www.cycling74.com – Max/MSP/Jitter site with forum, projects and a fantastic tech support

http://miscellanea.com/how-to-get-more-arduino-analog-inputs/ – brilliant multiplexer tutorial!

http://bildr.org/2011/02/cd74hc4067-arduino/ – another awesome multiplexer tutorial