[beagle3]

State of the art implementation, using random projection for reasonably accurate yet hundreds to thousands of times faster: http://radimrehurek.com/gensim/

Random Projection is something you should be aware of if you do any kind of large dimensional modeling. It is magic.

[vedrana]

Also, random indexing, which is in its theoretic essence the same thing: http://www.sics.se/~mange/random_indexing.html