Hope this hasnt been posted on already.Ran across this free applet which allows you to design your own bicycle using BikeCAD.I havnt used it so cant give an opinion on its usefulness.There is a pay BikeCAD also for the pro people too.Lastly there is some unique projects to look at built by the author.
http://www.bikeforest.com/about.php
http://www.bikeforest.com/about.php