Hi, Well its actually a tricky question to answer as there are a few solutions out there that are focusing a different markets.
StorMagic, where I'm from, do a 2 server solution called SvSAN which is designed for multi-site environments and essentially centralizes that 3rd server or Neutral Storage Host (NSH) to your datacenter. You can then have a single (or multiple if preferred) NSH acting as the witness to all of your sites thus reducing hardware requirement by 33%. You can do it with just 2 servers and no NSH at all but that is not advised as you could end up with a split brain scenario in the event of a failure.
ESG recently did a lab review on it if your interested in having a look: http://www.stormagic.com/whitepapers/esg_lab_report.php
There are some great Virtual SAN solutions out there and this may not fit what you're looking to do but when it comes to 2 server and multi-site environments, we consider them non competitive.
Hope the info helps
Steve