I built a cash forecasting app (http://www.money-stew.com) that I use religiously to verify that I won't have any trouble paying my bills.

I actually managed to get some traffic and ad revenue ($100< a month) when I first developed it and got it into the Google web app store and was featured for a little bit. I think there where quite a few bugs that I ignored and I stopped work on it for a long while.

I continue to try and improve on it, but it rarely gets the bulk of my free time.

Its been difficult for me to get _any_ feedback on it, so I bounce back and forth between feeling like its a worthwhile venture or its just a pet project that is useful to nobody but me.