I started TwitterCounter.com as a fun weekend coding project after my partner said he wouldn't have time to start on it until he had finished some other stuff. I couldn't let it go so started coding right away. We launched 6 days later. Not it is hugely profitable and has 2 full-time employees.

More importantly: I think almost every major internet success was started as a side-project. Yahoo, Google, Apple, the list goes on and on...