Well, if you liked Fallout 3 then you would LOVE Oblivion, it's made by the same people and it's the same exact style/gameplay (customizable character, main quests, side quests, weapons, items, etc.) it just that Oblivion is Medieval but you can do all the things you can in Fallout 3 if not more. You can cast spells and make your own and enchant items (if your into the whole "power-up thing, you might like that kinda' stuff) and toggle 1st and 3rd person. The only thing that's different is no V.A.T.S :P I got it just recently because I played it at a friends house and thought it was great and like I said if you were a fan of Fallout 3 you'd definitely like Oblivion. Another great thing about it is after I got so used to staring at destroyed buildings, Barron wastelands and rubble in Fallout 3, I played Oblivion and found that the scenery was full of Lush green grass, plants, and tree's and it Has some incredible landscapes like snow-capped mountains and forests so you would love the open world wandering in that. Another great game is Fable 2 that's open world too and it's really fun. :D
Borderlands and Red faction guerrilla are very good.Oblivion and Prototype are also good.But AC2 it is just amazing,it is not like a FPS but it is like Prince of persia with a big open world to explore.