Centralized virtualization (ESXi, XenServer) in a completely Windows-free environment

With vSphere 5, there is a vCenter Virtual Appliance, so you can have the management of vCenter without a Windows server. There is also a web-based configuration option, which can be used instead of the vSphere Client.

Neither option is feature-complete when compared to the Windows versions, but it may do everything that you need it to on a one/two host environment. Or, maybe you can get away with a single Windows workstation/VM for advanced config.

ganeti + kvm has worked nicely for us.

If you're looking for a recommendation, I've been quite happy with Debian Squeeze + Xen and the cli tools for management. I haven't found anything I couldn't do. Even live migration of dumU's between dom0's works from the command line.

Take a look at oVirt - web GUI that works in FF or Chrome, all of the nice enterprisey features