Secure way to run other people code (sandbox) on my server?

Question

I want to make a web service that run other people code locally... Naturally, I want to limit their code access to certain \"sandbox\" directory, and that they wont be able

Answer 1

chroot, jail, container, VServer/OpenVZ/etc., are generally more secure than running as an unprivileged user, but lighter-weight than full OS virtualization.

Also, for Java, you might trust the JVM's built-in sandboxing, and for compiling C++, NaCl claims to be able to sandbox x86 code.

But as Checkers' answer states, it's been proven possible to cause malicious damage from almost any "sandbox" in the past, and I would expect more holes to be continually found (and hopefully fixed) in the future. Do you really want to be running untrusted code?