I was first introduced to ArchiCAD 6.5 at university back in 1999. At technical college before we had been taught the basics of 2d AutoCAD, so seeing the 3d capabilities of ArchiCAD for the first time blew me away! I thought the Ghost storey function was amazing, and the rendering capabilities were incredibly realistic!
A little later I also purchased David Nicholson-Coles GDL Cookbook which really opened my eyes to the hidden potential. Very geeky, but still quite exciting!