I came across this interesting paper on the motivations of developers contributing to Open Source projects a few weeks ago – I’ve finally got round to reading it & it does to seem to cover all the main motivational areas I could think of and have experienced. Though i was quite surprised that there was such a high percentage of developers paid to work on OSS seven years ago!
One element that I don’t think was stressed enough in the paper was how the stimulating environment of an open source community itself is a motivating factor. I find that people who contribute to open source software tend to be highly motivated and passionate about what they do (not surprising really, since they are contributing..). This makes for a great environment to challenge and learn from each other, which is really hard to match in any other situation.