I often receive requests for the implementation of Explicit Semantic Analysis, so I thought I'll put together a Web page about it :)
At the moment, we do not release the actual source code, as we do not have resources to properly maintain it. Most of the algorithms should be covered in our papers. Below I list a few publicly available ESA implementations in the hope they might be useful for you.
By the way, my Wikipedia preprocessing code (WikiPrep) is available here.
Wikiprep-ESA (by Cagatay Calli). The exact settings needed to replicate our IJCAI'07 results are listed here. If I understand correctly, this code is suitable for the current (as of 2010) versions
of Wikipedia. If you would like to process old dumps (specifically, the 2005 dump I used), you probably
want to use this version, which you can download from this commit tree.