[Bullet Magnet]

I got another one, kind of both a like and dislike:

(Mostly 1st edition, but I saw it in the 2nd edition as well) The adventure modules. As a 'dislike' they are a bit light on details for areas in the module in question.
However, as a like, this allowed GDW to provide SOME info on a larger area. Each module was almost a mini sourcebook for the region it covered. Looking over some of those modules last year, I realized any one of them could theoretically be expanded into a campaign in its own right. Yes, that would require the GM to do some independent research into the specific region, which might not have been that easy back in the 80s when these were printed, but now with the web, a lot of that is much easier.
The real trick is just getting the time and the initiative to do it.