This week I have had the pleasure to meet Jan Wielemaker, the author of SWI-Prolog. I've been in Madrid in a meeting of an European Project I have recently joined in which Jan has worked.
Unfortunately he has done his part already and will be no longer involved in the project :-(.
That's a nice coincidence, back in 1998 I needed to do an immersion in Prolog. I used SWI-Prolog for that and since I was working as a proof-reader by then I did a careful revision of a couple of chapters of the user manual and sent it to him. He was very grateful and the corrections were merged.