Good Sandbox/Free Roaming game?

Just Cause 2

I don't see Just Cause 2 on your list. I've heard it's fun, though I've never played it myself.

Grand Theft Auto 4 Need for Speed Most Wabted 2013 Burnout

Batman Arkham City?

For mindless short burst of fun, Just Cause 2 is great.

S.T.A.L.K.E.R: Boku no Chernobyl + 2013 Modpack

Kingdom of amalur: Reckoning, The elder scrolls IV: Oblivion, Fallout 3, Red Dead Redemption, And dead island sounds good

Just Cause 2 is amazing fun as everyone has already said, it is a must. Besides that I might suggest Kingdoms of Amalur Reckoning and either of the Borderlands titles or any of the Fallout games.

Just Cause 2, any game from the Grand Theft Auto series, Retro City Rampage, or even Arma 2: DayZ Mod.

Project Zomboid, zombie sandbox survival

"Saints Row: The Third (Please don't recommend for fourth, Ima' prob buy it for console)"

Don't do it, mod support was announced, they are creating tools right now, gonna be so much better on PC.

Link

"THE FUTURE OF SAINTS ROW MODDING

"In case you’ve been living under rock, I got the green light yesterday to announce that Volition is now officially supporting modding and our forum at http://saintsrowmods.com for all the Saints Row PC games! Jeff Thompson, the Studio Director of Programming at Volition, is now working directly with us and assembling documentation, tools, support for all the file formats in the games.

Things are going to start happening this week with SRTT as something of a “test run” for the release of Saints Row IV. Saints Row 2 info is going to come later since Volition didn’t do the port and it will be a bit of work to actually dig up the info we need.

To the end user, this means that modders will now be able to create new clothing, new vehicles, new npcs, new weapons, new missions, and even world geometry that will all greatly extend the life of the game!"

Mindfart.

garry's mod best sandbox ever

Borderlands (or 2)

Fallout 3 and Fallout New Vegas are probably my favorite open world games but not really games you play in short bursts unless you feel like hunting mutants instead of completing quests. I usually end up playing for multiple hours once I start er up.

But yea Just Cause 2 is probably what you're looking for.

If you like car games I would recommend Test Drive Unlimited 1 (not 2). I play it just for fun, no multi-player, I don't engage in races or other stuff that you can do. I basically just drive around for 20 or 30 minutes when I want to take a break from work. You have a pretty big map, ok-ish driving model, good environments, and a good selection of cars, if you take into consideration that the game was released a few years ago. There are also some mods for it (on PC) if that tickles your fancy. But if you don't like driving games, just ignore this post :)

Titan Quest open world ?
LOL

I think you've covered all the ones I'd recommend already. :)
Here's an older thread with a ton of responses in case you feel like digging through forum suggestions of the past.

GTA 4/Episodes from Liberty City,if you have xbox 360 or ps3 Red dead Redemption,Just Cause 2,Assassins Creed Series,and probably Dead Rising 2 You can walk around in a whole Mall...or park idk what it is :D

Edit: How did I forget about Saints Row The Third <.<...

DayZ

U no have Just Cause2 and GTA?!?!
-Sleeping Dogs
-Gothic games(not 4th)
-Red Faction Guerrilla
-Assassin's Creed games
-many many others :P

just cause 2, and it also has MP beta tests on weekends announced here