CLHEP is a library of a few utility classes which represents what I would like to have in a future Class Library for High Energy Physics. Most of them have been taken out of a draft version of the MC++ Event Generator toolkit, hence they may contain some special features which are not suitable for a general class library.
The CLHEP is distributed and maintained by the Libraries for HEP Computing - LHC++ project at CERN. There you will also find the new CLHEP home page.