VMware recently released a free version of ESXi. Ideal for people that are running their own little home-datacenter ;-)
Rich over at VM /ETC blog wrote a nice article discussing the limitations of this free version.
As for me, the hard disk controller on the motherboard of my test machine is not recognized. I could obviously buy an external one to build myselves an ESX whitebox but I’m sticking with Hyper-V for my virtualization needs for now.