Which kernel level sandboxing system is the best?

Blaine writes...

Hey Allan and Chris,

First off, I wanted to say that I love TechSNAP.  I have been watching since almost the beginning, and it is one of the best podcasts that I watch each week (even though I am a developer and not a sysadmin).

I have been having trouble finding an answer to this question online, so I thought I would invoke the knowledge of the professionals:

In terms of density, which kernel level sandboxing system is the best?  We are targeting Linux systems, but even some more information on the different types of kernel level sandboxing would be very helpful.

Thanks a ton,