ThePEG has been broken out from the PYTHIA7 program to better factorize the parts which were PYTHIA-specific from those which are general model-independent components of the toolkit and which can be used by any event generator model. Consequently the current release of the PYTHIA7 program is built within the framework of ThePEG.
The current version of ThePEG is 1.2.0 and is available as a compressed tar file: ThePEG-1.2.0.tgz. For installation instructions, please consult the joint ThePEG/Herwig++ wiki pages. You may want to download the larger distribution, ThePEG++-1.2.0.tgz which also contains the PYTHIA7 distribution and some small test cases.
There is no up-to-date documentation available for ThePEG, but the basic structure is very similar to the earlier PYTHIA7 versions which are described in Comput. Phys. Commun. 134 (2001) 365 (available in HTML and postscript format. Proper documentation is under construction using the Doxygen documentation system. The current state of this is available here.
If you need more information, or if you have found problems or bugs in the current version, please send me an email. You can also have a look at the joint ThePEG/Herwig++ Wiki.