In case massive role-playing game Oblivion isn't expansive enough for you, the first official full expansion pack is on its way.
According to British gaming mags PC Gamer and PC Zone, the expansion will contain more than 30 hours of new gameplay with branching quest storylines, and the landscape itself will change according to the decisions players make during their adventures. There will also be new creatures, dungeons, and quests. Two of the new monsters include a Gatekeeper, a golem-style creature made up of stitched together body parts, and the Knights of the Order, which are made from tree roots.
Right now, it's planned on PCs only, but I wouldn't be surprised by a 360 version coming next.
GameSpot: Oblivion expansion revealed