The only reason I didn't pick up 4e D&D is because I have 1e, 2e, and 3/3.5e and I just refused to start collecting books for another D&D system. So, now Pathfinder is also my favorite. Plus I'm a huge collector of D&D worlds, which is why I have a full collection of Realms sourcebooks (except 4e Realms material, because I don't like what Wizards did to the setting, it isn't the same setting to me.), Planescape, Spelljammer, some Dlance, and most of the Ravenloft sourcebooks. Now, as I said, PF is my new obsession/collection.
I'm still active on a Realms board/site where some of the game designers and authors are willing to post to anyone who is civil to them.
But I have tons of other sourcebooks for Shadowrun, probably not 4e? Shadowrun since most of my material is from the 90's. Otherwise, I've picked up some GURPS over the years, mostly as material to read because they're fun to books to read.