Windows 98 SE (just for fun) - 128MB RAM
Windows XP Pro SP3 x64 (for nostalgia's sake) - 512MB RAM
Windows Vista Ultimate x64 (for nostalgia's sake as well) - 4GB RAM
Windows 7 Pro x64 - 4GB RAM
Windows 8.1 Pro x64 - 4GB RAM
Windows Embedded 8.1 Industry Pro (for making images for embedded systems) - 4GB RAM
OS X 10.6 Server (for Rosetta) - 2GB RAM
Elementary Luna - 1GB RAM
Ubuntu - 1GB RAM.
All these on a 27" iMac with 32GB of RAM. I only have 4-5 of them running at the same time though.