[malloyd]

Undoubtably the one I've gotten the most personal entertainment out of is Vehicles 2e, for 3rd edition. But as a freestanding toy more than anything - it's been almost useless for actual use in *GURPS*.

Most inspirational, I still have to give it to GURPS Voodoo - origin point of the system that's since turned into Path/Book Magic, Sorcery etc. and full of a bunch of other cool mystical stuff I at the time mostly didn't know.

Most useful now, tough call. I refer to Power Ups 2 Perks probably more than anything outside the core books (or I would if I hadn't done it so much I remember most of it without needing to...) but I think for most actually useful I'd have to nominate one of the GM focused books for one of the PDF lines - like Dungeon Fantasy 2 or Action 2 - these are quite helpful GMing aids well outside the lines they are specifically for.