[jcoglan]

As someone who runs a lot of his own open-source projects (see http://jcoglan.com), and occasionally contributes to others when I find bugs, the biggest reason I do it is to learn things. Not just to learn programming, and various problem domains, but to practise writing and speaking about my work, writing good documentation, and (crucially) practising API design.

Making something you're intending to ship to strangers (or to yourself writing future projects with your tools) forces you to write good documentation (if you want anyone to use it), which in turn forces you to hone your API design skills. If you're having trouble writing the docs for something, you probably haven't adequately solved the problem yet.

I'm not claiming to have done a great job at this on all my projects, but it's a learning process and occasionally my projects find a user base, which keeps me busy, provides new challenges when they find bugs, and if your code's any good it boosts your reputation as a nice side effect.