Guest account as sandbox? How much extra security do I get by downloading and running code only in a guest account. My normal user data is safe. Shared data is write able somewhat, but I can do a search for writable areas and cut these down

I want to download a tool but the more I read about the NSA and all their friends the less and less code I want to run on my box. I figure I can run as guest and that shouldn't have write access to much. Is there a way to run a normal GUI app in something like a FreeBSD jail?
In reality, little to none. The best result would be having less downloaded junk in your normal user account. 

OS X is sandboxed by default. Applications are by default run in a sandboxed environment without root privileges. Your best bet for security is trusting the sources you download code from, and being wary of code that asks for administrator or elevated privileges.