I've researched this stuff a lot in the recent months but i can't grasp any solid basis on how they achieve runing unsafe scripts in so many languages without a problem! I'm sure they are running virtual machines since language specific sandboxes won't cut it, but what about white listing every language? There has to be another restraint i'm not seeing.
Codepad explains how they provide security on their about page:
This seems like a reasonable strategy to me.
For more background on sandboxing technologies, see the following: What is sandboxing?, How can I restrict what an application can do with my computer?, Monitoring system calls (in a reliable and secure way), and Wikipedia's entry on chroot.