New verion 5.4 pre-built binaries and source code are available at Swi-Prolog Home Page.
Windows, Mac OS X, and Linux are suported. If you are into the Semantic Web, the good news is that the semweb library is built in and works great to read and logically query RDF and OWL files. Very useful stuff.
Swi-Prolog also has built in libraries to parse SGML and XML, built in socket servers, socket client libraries, CGI, HTTP support, ODBC support, and supports SSL.
Prolog is not always the best language for a task, but when it is, Prolog programs tend to be very consise - get the job done with relatively few lines of code. In a sense, Prolog is also a late binding language like Python, Lisp, and Smalltalk.
Swi-Prolog is licensed under the LGPL.