Generally speaking there aren't a whole lot of Sandbox RPGs out there right now, that's more or less Bethesda's thing. There ARE some though:
#1: Risen, I'm surprised more people have not suggested it. A spiritual successor to th Gothic games it's an Action-RPG that allows you to go anywhere and do more or less anything, though you do wind up needing to join 1 of 3 factions (Paladins, Bandits, or Mages) to determine a "class" and define how the rest of the game will go at one point. The storyline is okay, but not exceptional, involving mysterious ruins literally rising from the ground all over this island your trapped on while giant sea monsters and storms prevent anyone from leaving. Kind of cool.
Technically I think "Gothic 2" is a better game, especially with it's expansion (Gothic 3 was horrible though), however it shows it's age.
#2: Sacred 2, while the voice acting is a mixed bag, this is sort of like a version of Diablo 2 turned into a true sandbox game and pumped up on 'roids. The storyline is very weak, but diablo-type gameplay in a sandbox with towns and all kinds of little quests and such around makes for a very addictive gameplay experience. I spent a ton of time exploring this gem... it's highly underrated though, I'm guessing because it is a Diablo clone of sorts.
#3: Divinity 2, this is sort of like a third person perspective attempt at the diablo thing, it starts off a bit weak but gets a lot better. The graphics are a mixed bag, but generally tolerable (it's a current game). You have a pretty massive word to explore, a decent plotline (even if it leaves off at a cliffhanger), and some neat mechanics like controlling a flying castle, transforming into a dragon form, and collecting the severed limbs of defeated opponents to combine them into a monster you can summon.
It's a spiritual sequel to a game called "Divine Divinity" which is another free-roam sandbox "Diablo" type game, which became a cult classic upon it's release, but died due to a horrible sequel called "Beyond Divinity". The original "Divine Divinity" is a good game, but can be hard to find.
#4: Amulets and Armor: this is a freeware game that is actually pretty good when you can get past the horrible, dated graphics and controls. It was originally developed as Shareware but didn't succeed due to limited exposure, and eventually became freeware. A decent little time sucker, that probably won't cost you a dime.
#5: In the same vein as above there is a mecha-based Roguelike called "Gearhead 2" being a roguelike it has minimalist graphics but is a pretty deep experience with variable plots, and combat both in and outside of customizable giant robots. Quite cool.
Those are some suggestions if your looking at Sandbox RPGs as opposed to just good RPGs. They aren't the most common type of game out there.
