Can the best things in life really be free? Download Sun xVM VirtualBox and find out in less than 10 minutes.
Sun xVM VirtualBox software is the world's most popular open source virtualization platform because of its fast performance, ease of use, rich functionality, and modular design.
This is a PUEL (Personal Use and Evaluation Licenese) version. There is another version (OSE, Open Source Edition). The main difference is that PUEL has USB support in the virtual machine (if I'm not mistaken). You can see all the differences here:
(this page is also owned by Sun Microsistems)
And yes, VirtualBox have a Unity or Coherence mode, in that case called Seamless.