This is a thread safe, apartment neutral dictionary. It’s main use is that you can store it in the ASP application object without a performance impact. It also gets used all over the place due to having features like persistability (very easy to store into a database for example), recursion (dictionaries within dictionaries), and the ability to use almost anything as a key (including arrays).
A pre-built binary is bundled with the gnomon installer (available here), or you can get the code from subversion (see main page).