Does anyone know any decent sandbox games like Garrysmod, that you don't need something like steam for?

I had a working copy of Garrys Mod up until this morning, I went to play it, it closed on me and popped up a small ERROR window which told me, you did not pay for Garrys Mod!, all I could do was close it. and yes I do know that it's unfair to the people who paid for Garrys Mod that I had downloaded it, don't get me started on the freedom of information act, that's not what I'm here for. I am looking for PC games, preferrably offline, hopefully near as good as garrys mod, hopefully not Minecraft. Any links would help, I would really appreciate it. Sandbox style, Editable, Roam around a map at will/ non linear. On another note, I have a 64 mb graphics card, Garrys mod ran great, with almost all of the addons on it, and GTA San andreas runs great, with tons of mods, and a trainer, so I know there are some pretty decent games I can run.