Your most successful side project?
vdibart: Doesn't exactly fit your criteria in terms generating revenue, but I wrote a plugin for TiddlyWiki (http://www.tiddlywiki.com/) that allows you to store your address book inside a TiddlyWiki file.  It includes import and export, and since TiddlyWiki is JavaScript based you can fully script your address book to your liking.  It's called TiddlyWiki Address Book (i.e. twab, http://www.tiddly-twab.com/).  It's fairly popular among those who use TiddlyWiki and more importantly is something I'm proud of for a number of reasons.