The current version is HERWIG 6.202, of February 20th 2001.
Here is the source code. Unlike previous versions, two include files are needed: one and two.
Version 6.201 of November 27th 2000 can be obtained here: program and include.
Here is the documentation.
Here is a simple test job that generates QCD 2->2 events at the Tevatron.
There is also a test job that shows how to read in SUSY input files and generate SUSY events at the LHC. To run it you will need the file sugra_pt2.in from the ISAWIG web page. Note that it produces a few warning messages when the decay tables are processed. This is due to the fact that both the built-in decay table and the one read in from file contain entries for top decays, and is not a problem: the duplicates are simply deleted.