I just built http://wethr.ca - it only took 3 hours and $5 for a domain name. It's extremely basic, but extremely useful to me as-is. That got me inspired, that I could build something so useful so quickly. So please share your smallest, but most useful project - even if it's super buggy and nowhere near finished yet.
I threw together http://www.dragdropcomics.com/ to make it easier to create Rage Face comics on Reddit, then I realized that there were already two or three very good and fully fleshed out Rage Face comic web apps, so I kinda left it as is. My idea was that artists could upload their own character sets and people could pick a character set to make a comic out of, but I never really moved beyond the initial version of it.
http://coloringout.com - I'm not very good with colour schemes - this one let's me take an existing css and transmogrify it to a new colour, keeping the relative "distance" between each colour value. I built it in php and then in python. I use it....don't think anyone else does though :)
The following approximation of Adobe AIR took gluing together and/or modifying about a half-dozen existing open source projects with just about 100 lines of code: