Open source software has had a huge impact on the web and IT, with solutions like Linux, Apache, PostgreSQL, Wordpress, Firefox, Audacity, VLC, etc., becoming major forces.
However, in video surveillance, that has not been the case. The most well known open source offering for VMSes has been Zoneminder, which is Linux only and stopped development for 2 years.
Open Source vs Open Platform
Now, open source and open platform may sound similar but they are not the same. Open platform is typical in surveillance, implying that the software can work with third party devices (e.g., cameras, access control, intrusion, PSIM), without major restrictions. However, open source goes much further, making the source code for the VMS publicly available for re-use and modification.
Nonetheless, many in our industry still confuse the two - see IFSEC Global's unfortunate misreporting of Milestone open sourcing their VMS (Milestone is open platform, not open source).
iSpy Open Source VMS
Another open source VMS solution, named iSPY, has emerged. Not only is it actively being developed, it runs on Windows machines, making it easier for most consumers / low tech users. The two of them make iSPY a very interesting VMS to consider.