A great overview of the features of the different SharePoint 2010 editions
The following document gives a good overview of the features available in the different SharePoint editions: Foundation (free), Standard and Enterprise.
http://sharepoint.microsoft.com/en-us/buy/pages/editions-comparison.aspx
It is good to see the power that is available in the free SharePoint Foundation edition, so there is no reason to not use SharePoint as a foundation for you collaboration applications.