Spoke Calc

I’ve got this handy dandy Excel spreadsheet that lets you calculate
the spoke lengths for various Hub/Rim combinations, as well as input
the measurements directly. http://www.sheldonbrown.com/rinard/spocalc.htm

Also, this page is great for learning to build wheels. Many a novice
(myself included) has built their first wheel following this
tutorial. http://sheldonbrown.com/wheelbuild.html

Matt