Prolog was the language I used for my diploma thesis, back in the wild 80's (last century, internet had not been invented). I coded nearly everything in Prolog, rule processors, expert systems, user-interfaces (a real nuisance in Prolog, believe me!) and a persistence engine... (yep: store things, back then you had to do it on your own). Great fun, unstable implementations on the first commercial Unix machines.
As a reminiscence to that "wild age" - Learn Prolog Now. Contains links to several implementations (too bad they omitted XGP, one of the fancy Mac versions).
Btw: Prolog is much more fun than the current WS-* debate...
