I built Artpip (https://artpip.com) whilst working fulltime. Spent the odd weeknight and some weekends developing it over the course of a few months, then the same again when I added the pro mode once I realised it could actually make me some money.

Not sure you could classify it as a startup, but it makes me a few hundred dollars per month. I'd actually love to grow it more but I'm not sure what the best direction to take it in is. If anybody has any feedback or feels like getting involved, feel free to drop me a mail at "tom @ artpip.com"!