I hate answers like this. This answer is just as useful as answering the question of "where are we" with "in a car." A http://justfuckinggoogleit.com/ would have been more helpful.
When I googled for it, I found a couple of other links:
http://nepomuk.semanticdesktop.org/xwiki/bin/view/Main1/
http://en.wikipedia.org/wiki/NEPOMUK_(framework)
But they still didn't answer the question that I wanted to know: Should I run it? What's it good for? What can I actually do with it today? After 5 minutes of reading, I came to the conclusion, that whatever benefit it might
bring in the future might be great, but I couldn't see it do anything except consume resources.
I turned it off.