Has anyone ever seen The Website is Down? In the first episode it looks like the web dude is effortlessly switching between at least 2 different operating systems with a simple alt-tab (I think one was Fedora Linux?), what is he using? Something like VMWare?
Looks to me like VMware Fusion running with Unity enabled. (You can see the little VMware logo on one of the windows.) Unity will display windows from the VM in your native window manager instead of the entire VM in its own window.
Necroing this question because I love that video and there aren't any right answers yet. ;-)
The "Web Dude" character is using Mac OS X as his primary operating system, and is using Microsoft's Remote Desktop Connection software (the old version) to connect to a Windows machine, which in running a number of guest systems in VMware (not Fusion, probably Workstation), and making additional VNC or RDP connections.
You can see the first layer of this in the first 30 seconds -- when he answers the Skype call and switches back to Halo, for a moment you can see the Mac application switcher (command-tab), and all of the programs he has open: (in order) Skype, Firefox, Halo:CE (the Mac port), Audio Hijack, Remote Desktop, Grab, (Finder), and Terminal.
The second layer is only clear at a few points in the video -- one point is at 4:05 when he's messing with the Exchange accounts -- he's got Outlook open in the foreground of the windows machine, with VMware running in the background (you can also see it in the taskbar). The "webserver" is one of the VMware guest OSs displayed in tabs at the very top of the screen. The VMware tab bar also shows up at the very end, about 9:45.
This means he can use command-tab to switch in and out of Skype, Halo, and Remote Desktop connection; and from within RDC, can use alt-tab to switch between applications or remote connections made from that machine while still being able to command-tab out to mac at any time.
It's a bit ambiguous how the windows RDC machine often changes state when he command-tabs back into it from the Mac; it's possible that there's video editing going on, or that someone else is driving that machine with a directly-connected keyboard and mouse whenever the connection is obscured by Halo.
The creator himself explains the setup in this interview.