Although originally a commercial product, a new, open source version, Free Poplog, has been made available for no cost. It runs on a variety of Unix and linux platforms as well as Windows 95/98/NT/2000
An excellent resource for Poplog can be found at http://www.poplog.org/. As of 2011-01-10, the domain is for sale.
A more up to date source of poplog, along with documentation, additional packages and a version packaged for linux+PC can be found here (at the University of Birmingham): http://www.cs.bham.ac.uk/research/poplog/freepoplog.html