Wikipedia
Anyone ponedering the 'What's in it for me?' question should look at Wikipedia. It's an entirely open encyclopaedia effort - even open to the extent that everyone has edit rights! Nobody is 'incentivized' to contribute, yet there are 233112 articles, many in multiple languages and the quality (from my random tour) ranges from good to excellent.

One thing though: there's no entry for "knowledge management" - maybe we're all waiting to be offered air miles before we'll contribute?

[thanks to Dermot Casey, Project Manager at GE Consumer Finance for making me aware of thiw Wiki via AOK]