Hey, yeah sure! Erm, let's see. So I use libvirt and QEMU/KVM, where libvirt is essentially an api that makes interfacing with QEMU and KVM more consistent and nicer.

Then I use virsh (command-line) and/or virt-manager (gui) to create virtual machines (these in turn talk to libvirt). The machines kan be anything Qemu supports, so you can emulate PA-RISC, Alpha, Mips, ARM, i386, AMD64, you name it. You can then install OS'es on them. There are some freaks out there that have been running Windows NT MIPS and HP-UX 10.20 for PA-RISC on it.

Most of my machines are doing KVM, which essentially is hardware-assisted virtualization. This requires the VM to be AMD64 though.

I currently run 10 virtual machines:

castel (rhel6) - chimera (chimera-linux) - dragon (dragonflybsd6) - falcon (freebsd13) - gygax (guix) - haiku (haiku) - nomon (netbsd9) - seven (win7) - square (winnt) - xenon (alpine)

You have a gui to these things (and SSH of course) through Spice. With the right drivers and agents (not available for all operating systems) you even have automatic guest display resizing, mouse in/out, copy/paste, etc.