Old books used in the new game

[Thurbane]

I've used some of my immense 1E/2E AD&D book collection in my 3.5 games, but really only for inspiration or maps...

 

[Mouseferatu]

Lots of campaign stuff, certainly. I've used (or at least wanted to use, and was prepared to use) material from Ravenloft, Al-Qadim, Dark Sun, and Planescape. I've used material from the 2E Book of Artifacts (which inspired the Tome of Artifacts I put together for Necromancer and Lion's Den), and several of the new spells I've introduced in various Lion's Den products were inspired by material from 1E and 2E.

So yeah, quite a bit, though little of it mechanical.