I once talked about this, but through Kelly Lab Blog I find that the best source, which is constantly updated and has a good comparison of GIS alternatives both free and private is this Wikipedia page.
It has comparisons of functionalities and support of operating systems such as:
Online Guides (Ai / no / viewers)
Open source (if not)
Works with Windows (Yes / no / under java environment 7 ActiveX)
Works with Mac (Yes / no / under java environment)
Powered by Linux (Yes / no / under java environment)
Works under UNIX Berkeley (Yes / no / under java environment)
Works with UNIX (Yes / no / solaris / CLIX / under java environment)
Web options (Yes / no / DHTML / other options)
The valuable thing is that each link has information related to the software ... and what are they all, now I see that Wikipedia supports Javascript options to sort the table.
Additionally there are map servers and map caches
