Tripwire is an old and well-known security vendor focused on host intrusion detection (specifically file integrity checking).
Over one year ago the company saw an opportunity in virtualization and joined the VMware Technology Alliance Partner Program.
In June 2008 they release a free tool (and a blueprint developed in collaboration with VMware) to verify the ESX configuration.
The product was so successful that Tripwire must be in the process of reconsidering its corporate mission. Today in fact it was announced the recruitment of a well-known virtualization expert: Stephen Beaver, former System Engineer at the Florida Hospital in Orlando.
Beaver will work at Tripwire as Virtualization Evangelist and will write on a new blog that the company will formally launch on Dec. 15.