Someone posted this link on another modeling site, and it's pretty nifty. Although it's intended for real life construction, a lot of the stuff is also very useful for modeling:
http://www.blocklayer.com/default.aspx (http://www.blocklayer.com/default.aspx)
Just the thing to possibly eliminate some of the trial and error involved in making multi-valley roofs for a start. I've bookmarked it!