I will add to this, for those who already have Mac Minis with the Optical drives.

http://www.mcetech.com/optibay/index.html

This is a chassis that allows for a second hard drive in place of the Optical drive in Mac Minis.

MCE Technologies also makes chassis that allow to put a second hard drive in a Macbook or Macbook Pro. I use these to put SSDs in my laptops.

Other World Computing also has videos showing how to install these.