Aside from a host of fan-made and free content, there are currently 16 active publishers who licensed their products for conversion to Fantasy Grounds. This resulted in 86 new add-on products getting produced in 2010 and 17 so far in 2011. The system also has the most reported users with over 20,000 licensed users to date.
There are 39 systems in total supported by either a commercial or a fan-made ruleset or extension to date. You can see the list here: http://oberoten.dyndns.org/fgwiki/index.php/Category:Systems