I'm into collecting obsolete electronic games and hardware. I have alot of Iomega's storage devices. (Iomega was/is famous for developing new products that are three years too late.)
I try to keep up with the latest and greatest in programming languages. That also means that I tend to collect books that quickly become obsolete. (kinda like phone books, but even Mythbusters
found a use for them..)
I collect 60s/70s era soda bottles, specializing in bottles that are marked Sugar Free and/or celebrate the Bicentennial.
I also collect cookbooks, but I don't think that comes as a surprise to anyone.