A project I've been meaning to do for a long time, but more serious stuff always seemed more important. So I just decided to spend a decent chunk of my free time on getting it done and released.
It's a little drawing app for iPad and iPhone, if you wear Red/Blue 3D glasses while using it the lines will appear in the third dimension. It has two modes: black lines on a red blue grid, and a dual red/blue line on a plane background.
You can share drawings as images in the usual UIActivityViewController-way but my favourite features are that the lines are editable for stroke width and (in dual-line mode) depth/height from the page, and that it uses state restoration to make sure you're last drawing is always presented to you each time you open the app, even if it's terminated in between.
I think it's fun, could be cool for kids etc... though the real project was writing it well under the hood and making it a playground for good coding practices. It's not going to make a million pounds but it makes me happy :)