Any opensource equivalent of virtual center?
It's not a GUI, but I know libvirt is able to talk to ESX machines.
VMware Go allows you to do some fairly limited ESXi centralised management. There are some compromises and none of the advanced features (no cloning, deploying VM's from templates and VMotion\DRS\HA etc are totally out) but it does provide a single console for powering on\off and some other basic functions. The main drawback is that it is a cloud service.