Fallout 3/Oblivion/Fallout: New Vegas are the perfect sandbox games for me.
Strangely enough all have connections with Bethesda...
I don't think games like Fable II should be classed as Sandbox, they have many restrictions as to where you can go and require a more linear based game. Still a good game but in a slightly different way which is not sandbox.
hmmm... Oblivion and Fallout 3 in different categories... strange. Maybe it is because there are no goblins in Fallout 3.
On a side note, Minecraft is a good Sandbox game, maybe the only "true" sandbox game. True meaning there are absolutely no restrictions as to what you can do within the games limits. You can't do lots of things like Fallout games, but you can immediately do everything within the games limits.
Like if Fallout 3 was like Minecraft as soon as you got out of the vault you could just pick up a Fatman and blow up GNR. Obviously it has to be restricted because of story/gameplay reasons, but since Minecraft has no goals and no story in an open world. It therefore does not have any limitations. So it is a good sandbox game in a different way.