Click here (version 2.6, Aug 13, 2003) to download a gzipped tar file containing the FeynDiagram source code and documentation. You can unpack the file on most Linux/Unix machines by doing:
  tar xzf feyndiagram_26.tgz

Important Note:
Users of GCC 3.4 report to me that they get the following error when they try to compile version 2.6:
  fd.h:231: error: `void FeynDiagram::init(page&, double, double, double)' is private   fd.h:358: error: within this context
I believe you can work around this by moving line 231, which contains:
 void init(page &pg,double xmin_in, double xmax_in, double yref_in);
to line 244 (i.e. from the "private" section to the "public" section). There are probably more proper ways to fix this, but I'm not going to get to it until I can find time to upgrade my copy of GCC.

See the README_compilernotes file included in the download for additional information on getting FeynDiagram to compile.