Now both SWI-Prolog and XPCE are LGPL

Jan Wielemaker has for years produced one of the great Prolog development kits. I know that to some programmers, Prolog seems like an esoteric language, but for some applications it is probably the best language to use; for example, these application areas are good candidates for prolog development: graph theory, search, natural language processing, constraint satisfaction. There are good free Prolog tutorials on the web, and if you really like the language, I could recommend the book "The Art of Prolog" (actually, there are many fine Prolog books - that is just the one I have purchased most recently).

XPCE is a very cool GUI cross platform platform that is implemented in Swi-Prolog. Jan has written some very interesting applications using XPCE, like Triple20 (RDF/RDFS/OWL visualisation and editing tool).