Calculating pulley sizes

Not exactly open source but I use this now an again around the shop:
http://www.owwm.com/Math/ArborRPM.asp
http://www.csgnetwork.com/pulleybeltcalc.html
http://www.owwm.com/Math/ArborRPM.asp
http://www.csgnetwork.com/pulleybeltcalc.html