Running code in a VM

The best way to run untrusted or untested code is on a replaceable machine or a virtual machine.

VMWare
Runs on Windows or Linux
Can run Linux, Windows, *BSD, or anything that runs on x86 processesors

User-Mode-Linux
Runs virtual Linux box on your Linux box

Both
Virtual machine can access host filesystem / network / devices if desired
Have independent filesystem in honking files or partition
Can have filesystem changes done separate from filesystem itself.
Great for kernel development 
Crashes are unimportant
Reboots are fast
Restoring filesystem a breeze