Just released: Bayon, a simple and fast hard-clustering tool, with support for repeated bisection clustering and K-means clustering.
Feed it a list of documents, optionally with weighted terms, ask for any number of groups you want it to output, and Bayon will do its best to assign documents to groups.
That kind of tool can bring a lot of benefits to Ning-like web sites, forums, etc.
And it seems to work just as advertised.
Yet another gem by Mixi.