XenClient. I would really love to have some minimal OS HyperVisor running, and then you slap multiple OSes on top of that w/ easy full GUI switching via some hotkeys like Ctrl+Shift+F1. Additionaly, special drivers to virtualize Gfx and Sfx devices so every VM have full desktop capabilities and low latency.

Unfortunately, it died because its very niche and also they couldnt keep up with development of drivers for desktops.. This is even worse today...