A popular use for the HP microserver is to use it as a cheap Synology NAS alternative. The OS used on Synology devices is based on linux, and is available as on open source download. In 2013 it was made installable for other hardware than that sold by Synology...
Most discussion about the development takes place at the XPEnology forum, in various languages.
ESXi images are available, but it can also be installed directly onto the hardware. Especially the N40L is well supported.