I've been running games for D&D 5e since it's release but before that I was actually running quite a different game called Call of Cthulhu. I used to think it was rather nerdy with goblins and dragons but after receiving the Monster Manual from a friend and seeing just how much was in there, I haven't looked back since.
These days I usually try to find some games to play in and I've expanded my books extensively - Ars Magica, Battletech, Delta Green, Dark Heresy, Deadlands, Eclipse Phase, Numenera, The One Ring, Runequest, Traveller..... I haven't even played all of these yet.