I've used this. It's interesting for a while. For a long time I had wanted to participate in David Lenat's Cyc project which set out to do the same thing, except they didn't involve massively volunteered participation. They were having a few highly trained and trusted volunteers do it all, and it was crawling along.
Well, I guess Open Mind is at least trolling the web for words that show up together a lot (one of the activities is to explain the connection between two words it has found on the web) but I don't know if it's doing more semantic analysis of web information.