Let's see what has not been discussed here: (my openness scores come from TVTropes's definition of sandboxes. The authoritative source as always.) Sandbox RPGs: The Saints Row series (which has been getting better than GTA in the "fun" category) Modability score: 5/10 (no official editor, but uses a XML language. New meshes are at the moment not possible though.) Openness: 6/6 Prototype. Yea. Haven't played the second one. Modability score: 3/10 (texture replacements?) Openness: 5/6 Space games: Freelancer (nothing out there is quite like it. Evochron maybe?) Modability score: 7/10. Google "discovery freelancer" Openness: 5/6 The X series games. Modability up the wazoo. EVERYTHING in the game (including basic attack/trade commands) is script based. 300-page scripting handbook, straight from the devs. Editors out there (provided links are still working) for everything imaginable. Modability score: 9/10. Just as moddable if not more than TES. Openness: 6/6 (unfortunately I could never get into Evochron. Whether it's the ultra-realism or 90s interface, I don't know. Great game though). Action RPGs: Torchlight (need I say more?) Modability score: 7/10. TorchED. Openness: 4/6 Titan Quest (current playing this after being disappointed with Diablo 3) Modability score: 6/10. Many official tools, and most everything is figured out by now. Applying multiple mods can be a pain though. Openness: 4/6 Simulation: Simcity 4. Don't bother with Cities XL (anything). Modability: 8/10. Most comprehensive transportation system of any computer game (short of real life) courtesy of Network Addon Mod. Also, CAM. Openness: 6/6. The very definition of sandbox (you start with freaking sand everywhere). The Sims. (really? you serious?) Modability: 8/10. Let's just say that it's very difficult to learn HOW to mod for this (prepare to be quite familiar with visual c++), but google "Awesomemod", "Nraas" or visit modthesims.info to see just what people have come up with. Openness: 6/6. --- Oh yea, all these games (except for Prototype) I have tried to mod in (however trivially), and my modding experience started back with Morrowind. So yea, that goes a long way.