Good Sandbox Games

Garry's mod

SimCity 4, SimCity 3000.. Hmm. Don't Starve!

The Sims and The Sims 2, The Sims 3 too although I prefer the older ones - nostalgia.

www.google.com

Fallout 3/New Vegas, Oblivion/Skyrim, Saints Row 2/3, GTA 3/4, and Assassin Creed (kinda) are all ones I like.

Rage! However I dissapointed on this game becouse of it's lengh, however it's worth to play it

tropico 3 or 4

Depends of the defenition of building, towns and game of dwarves are good Sandbox games, where you can build you're own town or undeground dwelling (Both strategy and Resource management games in a way).

A good Sandbox game (you can't build you're own creatons, but you can build facilities and complexs out of them), is X3 series, if you're into space games., thouth it takes quite a lot of time to get enough funds to build factories.

Minecraft.

saboteur

Just get Mount & Blade: Warband. I've put 123 hours into it and I'm still playing. Wait until it goes on sale again and you can pick it up for $5.

Just Cause 2, Red Dead Redemption (sorry not on PC ;d ), any GTA, TES/Fallout (if you want more a more RPGish experience), Fable 1 was fun, any Legend of Zelda (if you don't mind retro gaming).

Meybe its not the best, but Ive enjoyed every hour spend in Battlefield 2 mod Sandbox.

Just Cause, GTA, TES, Fallout, Sleeping Dogs...(too many games)...

how do you build things in TES, gta, zelda or assassin's creed? you are able to buy things for your char or buying new shops in ac, but you aren't really building

Garry's Mod

you might want to try out this one:
http://forums.tigsource.com/index.php?topic=26591.0
it has been released just yesterday! kinda looks like terraria but it's turn-based, you will see! check out urgent (it's free btw)

also, nice (i guess they've been mentioned here awready): dwarf fortress, from dust
also , have to mention that minecraft is MUSTHAVE

hope you enjoy :)

I have to say, Garry's Mod is not worth it.

It's a good game and mods make it all the more fun, but if you go in a server? Error city.

Anno 2070 and 1404, Minecraft.

Anno 1703 and 1404. Also Settlers, try IV or the newest one

Tetris

Wait nobody said GTA IV?

Why build houses when you can create galaxies? Or destroy everything with black holes?

Universe sandbox

Lego. The original shit. None of them ripoffs.

I see that you already bought Don't Starve.. Damn this game is epic :) I love it.

Hmm not sure what to suggest more if you really like building, maybe you should really consider to buy minecraft !