I use search engine software from Verity who implement machine assisted categorisation in a workbench tool such that the output keyword net can be applied to content as it is indexed. This works well in a corporate environment where content doesn't change that much and you just want to locate it in a defined categorisation structure. Verity also have a 'social network' product that allows people to see locate subject matter experts. I haven't worked with this bit yet but the demo looked cool.
I have also looked at Autonomy who popularised Baysian techniques for clustering results. Their search engine works really well for newsfeeds where the clustering is generally unknown and fluid. The search results can appear really random until the internals have caught up with a new cluster of information. I am told that the BBC News website uses this technique to create the 'related stories' links on it's website.