Version 6.510 was released on the 31st of October 2005. The HERWIG source code can be obtained here.
If you use HERWIG, please refer to it something along the lines of:
HERWIG 6.5, G. Corcella, I.G. Knowles, G. Marchesini, S. Moretti,
K. Odagiri, P. Richardson, M.H. Seymour and B.R. Webber, JHEP 01 (2001) 010 [hep-ph/0011363];
hep-ph/0210213.
Copies of these papers can be obtained here: hep-ph/0011363;
hep-ph/0210213.
Furthermore, if you use any supersymmetric processes, please refer
also to:
S. Moretti, K. Odagiri, P. Richardson, M.H. Seymour and
B.R. Webber, JHEP 0204 (2002)
028 [hep-ph/0204123].
A copy of this paper can be obtained here:
hep-ph/0204123.
The classic paper on HERWIG, although by now very out of date, is:
G. Marchesini, B.R. Webber, G. Abbiendi, I.G. Knowles, M.H. Seymour
and L. Stanco, Computer Phys. Commun. 67 (1992) 465.
A copy of this paper can be obtained here.