I am considering building a new desktop computer soon, and instead of installing multiple OSes in multiboot, I would like to have a lightweight linux OS installed, whose ONLY job is to run virtualization software (like VirtualBox)which would contain my OSes. Advantages over multiboot would include:
- No need for partitioning
- No wrestling with compatibility issues
- Having multiple OSes running at the same time (big one)
- Being able to internally network the machines.

Could anyone recommend a very simple, basic, and if possible free distro that I could accomplish this on?

How light are you talking about? Just a lean distro, or something that would be more at home in an embedded device?

If you are looking for a light distribution that can run a VM, check out Slackware, Debian, Arch, etc. You can only install enough of the system to get the VM running, and leave everything else out.

I'm looking for something lean (without all the bells and whistles) that won't hog up system resources. It does; however, need to be powerful enough to run probably two installations of Windows, one Ubuntu, and an installation of some form of BSD (not necessarily ALL at the same time). I will be using a six-core processor @ 3.2 GHz and about 10GB of RAM.

something lean

Arch , but there is a VERY big and VERY steep learning curve
Arch like LFS is not recommended for new linux users

but Arch is VERY customisable

I believe I might try Slackware or Debian. Thanks.