I have a few sites on the web and have had many good experiences with buying asp and php scripts from vendors.  The best companies are too numerous to list but beware: scammers (especially from 3rd World countries) are lurking everywhere.

Just one example of a  webmaster script scam pops in my head rightaway.  It is  a script written by a company called Jilmail, a Gmail/Hotmail/Yahoomail clone php script.