The Last Remnant
It's quite cheap for a pretty good game :D

Spec Ops: The Line, Bioshock ONE ONLY, Batman's, Assassin Creed's & Mass Effect's.

I'm still pimping Defender's Quest. Surprising fun game.

Gothic, Fallout: New Vegas, Dark Messiah of Might and Magic

Spec Ops The Line.

Bastion. Wait, it's pretty short.... Bioshock, Bioshock II, Half Life... AND NEVER SHALL THOU FORGET. Star Wars: KOTOR

F.E.A.R.

Haven't seen these mentioned yet but if you like GTA IV try Saints Row 2 and Saints Row: The Third. They're like GTA but fun.

I wrote in this thread and i thought i submitted my comment but i can't see it(maybe i'm stupid) btw, I would reccomend Batman Arkham City and L.A.Noire, the story length it's enough, are 2 great games and are open world(maybe the open world side of the game it's not their main part but is still good).

Alpha Protocol, Thief 1-3, Aquaria, Avernum: Escape from the Pit, Geneforge 1-5, Cargo: The quest for Gravity, Naval War: Arctic Circle, X3: Terran Conflict/Albion Prelude, Hotline Miami, Majesty: The Fantasy Kingdom Sim, Warlock: Master of the Arcane, Spec Ops: The Line, Elven Legacy, Sacrifice

Also, for non-steam games:
Planescape: torment, System Shock 2, Romance of the Three Kingdoms, Sanitarium, Ultima 7, The Real Texas, Might & Magic 4-5

I recommend FTL, but if you want an open world single-player game, I suggest an Assassin Creed game, although I have not played the latest one, I think that Brotherhood is the best one

Some good long open world games, that you can play solo:

• "Dead Island". GTA Zombies, with multiplayer.
• Anything "Elder Scrolls"
• "Legend of Grimrock". If you like Dungeon Crawlers.
• "Binding of Isaac". Rogue game, that will consume your soul.
• "Star Wars Knights of the Old Republic" 1 & 2
• "Max Payne" 1, 2 and 3. They are all pretty decent.

I recently started Stalker: Shadow Of Chernobyl and I must say, its got me hooked. It's open world, singleplayer (I think there might be a multiplayer too, but it's singleplayer-focused). It reminds me a lot of what I've seen of ARMA (never played it, but I've seen a bit of gameplay)

Dark Souls +200h

FUS RO DAH time, .... there is a lot of good single player game, mostly stragery and rpg, i suggest you chose any bioware if you like rpg,

Lego LOTR is awesome!
It feels like an openworld ganme when your running round middle earth looking for the next level, but there is pleanty of things to be doing on the way

Just cause 2 is pretty fun, i have had it for ages and never actually finished it cause i screw around whenever i play it.

Psychonauts's nice :]

Bastion's pretty good, as is BioShock.

Cave Story + if you like that kind of game.
Dark Souls if you like that kind of game as well.

Oh, and To the Moon if you want a great story ain't too much of a game ;)

Binary Domain, a very good single player game, and it also on sale too

NEW VEGAS

its awesome

Knights of Honor, Star Ruler, Europa Universalis, Crusader Kings, Victoria, King's Bounty, X-series, Space Rangers, Tropico, UFO: Afterlight, anything Total War, Railroad Tycoon, Port Royale 2, Patrician 3, Rollercoaster Tycoon, Uplink, Risen (1), Depths of Peril, Endless Space, The Guild. To name a few.

Mafia.

Singularity, Bioshock, Sleeping Dogs, Metro 2033, Dark Souls, Fallout 3

Singularity, Bioshock, Sleeping Dogs, Metro 2033, Dark Souls, Fallout 3