I used to enjoy hacking with Prolog, and I used Swi-Prolog when experimenting with the semantic web, in the sw’s early days. My only real problem with Prolog is that I have only been paid to use it for one intense six week project and a few one or two day gigs. I compare this to getting lots of work with Common Lisp, Java, Ruby, Haskell, Python, etc. Contact me if you have Prolog work you want done :-)

Swi-Prolog really is an amazing ecosystem.