good sandbox/free roam games

Heavily modded Fallout 2, FNV or Minecraft. You don't actually need much more besides Minecraft, too bad the official mod support gets delayed forever. Hell, they wouldn't need to do any new content ever again.

Fallout 3 and Fallout: New Vegas
Elder Scrolls series
Just Cause 2
S.T.A.L.K.E.R. series
Far Cry series
Crysis
Gothic series

Gothic 1 and 2. Risen.

Give Sleeping Dogs a go, one of the best free roam games lately.

I think nobody thought about this: Jak 2 and 3 are freeroam games.

Not yet mentioned, but I feel it's close - Freelancer.

Free Roam: All GTA games, Saints Row 3, Just Cause 2.

Sandbox: Garry's Mod, Minecraft, Terraria.

Funny that no one mentioned it, STALKER.
The first[ Shadows of Chernobyl] and the last[ Call of Pripyat ] are better than the second and you should skip it because it's kinda bad...

Also both game also have a huge modding community that made a variety of mods that change the game in many ways, making it realistic, hard or whatever you want!

Apart from Gta/Saints Row/Just Cause/Mafia(M2 with free ride mod) i enjoyed The Saboteur,Prototype,Assassin's Creed,a lot of rpg games(The elder Scrolls/Gothic/Risen/Fallout),Scarface,Dead Island,Burnout Paradise,FUEL(biggest map ever to be put in a game~14.000 square km),Far Cry3,Red Faction Guerilla,i actually enjoyed The Godfather 2 because of the gang empire you build in it,Mercenaries 2 is a fun game too,Stalker,......and Batman Arkham City of course

Scarface: The world is yours! <--- guaranteed good times!

Mount&Blade and M&B:Warband

If you have an old gamecube ps2 or xbox, look for a copy of Hulk: ultimate destruction (its b/c with xbox 360 too)
Probably the most fun you can have while shirtless in ripped jeans.
It's by the guys who made prototype, a lot of hulks gameplay style carried over into the later games

Old, but never cited when talking about sandbox/free-roam games : jagged alliance 2, a great mix of rpg, squad based turn by turn combat, free-roam strategic map, a great choice of mercenaries to hire (and they are not just a name and stats, they have personality) lots of humor.
From what I read the recent remake is rather bad.

Mobiloid ;) though it`s not a sandbox in a usuall understanding :D
http://www.mobiloid.com/

And of course:
Just Cause 1/2,
Prototype 1/2,
Saboteur, Batman AC,
Far Cry 3 (but i prefer just cause 2),
Sleeping dogs (but only if you like tpp fighting games)

Regarding jagged alliance 2- i love that game but wouldn`t call it a sandbox/free-roam game.

Fuel. And that's all you need.

Just Cause, Sleeping Dogs, Mafia, Grand Theft Auto, the Godfather, Scarface, Batman, and Saints Row.

Garry's Mod is a gem for 2.50 at a sale. Great game. I have played 230+ hours

I love the Grand Theft Auto series.

Wait for Watch Dogs to come out this year, it's going to be really really amazing.

Prototype, Saints Row (I recommend the second one), Just Cause 2 with mods, Sleeping Dogs!

Red Faction Guerilla and Fallout 3 get my vote

dead rising 2 is similar to a free roam game not as big a world as other games tho, some RPG type games which allow you to do missions in any order and have a small amount of free roam include the mass effect series and deus ex human revolution.

Minecraft is probably the greatest sandb

Though GTA: San Andreas, Saints Row 2 are excellent. GTA 4 wasnt very good imo, nor was SR3.