What motivates people to work on opensource development?

Some background on this post.

It is a discussion on the AGI (artificial general intelligence) or OpenCog mail list regarding the use of virtual credits. One of the late comers to the project (Yan King Yin) feels that given the effort that needs to be put in for AGI development, some sort of compensation beyond bragging rights, would motivate him better. He proposes the adoption of virtual credits, and the OpenCog project leader, Ben Goertzel gives his reasons why it might be a bad idea.

For anyone who has wondered what motivates people working on opensource software development, this discussion provides some interesting insights.