
I'm looking for a solution where a guest VM can "integrate" to the host, e.g. I can start Internet Explorer 6 if I needed from the guest VM. I think VMware Fusion has something like this - are there any free/open source solutions for this?

I'm looking for a solution where a guest VM can "integrate" to the host, e.g. I can start Internet Explorer 6 if I needed from the guest VM. I think VMware Fusion has something like this - are there any free/open source solutions for this?
VirtualBox has support for seamless windows with the appropriate Guest Additions, which are supplied for Windows guests. The guest menu bars (such as taskbar and Start menu) are nested inside the host desktop.
VMware Player (free) offers Unity mode which is the "seamless" app running you're looking for. Available for Linux and Windows.