SharePoint Foundation is a free version of Office SharePoint Server. Is it really free? The answer I think really depends on the use case.
If you are planning to use it as an intranet and have your users authenticated in your Active Directory then your licensing need is indirectly addressed as you anyways will have a valid windows server license.
If you are planning to use it as extranet or as an external website and are wanting to provide access to your anonymous users, customer and vendors then you will have to look at Microsoft licensing as you will be either wanting CALs or Windows Server External Connector license.
So is it really free? I think so - as long as you are properly licensed for Microsoft Windows Server and are wanting to use it as an intranet. Having known that the next question is the have and have-not features of the product. By far the best compilation of comparison i have seen on share point is here.
http://blog.blksthl.com/2013/01/14/sharepoint-2013-feature-comparison-chart-all-editions/
In my experience it is a great free tool for document management and collaboration. In fact our entire practice and project repository resides on a share point foundation implemented with a little customisation.
