A few people have mentioned wanting to join leagues. There are a few in the area, but they are hard to find. I'll post links here as I get them
Our own Andy started a league due to frustration finding other leagues to join. They had 8 teams or so for winter 2008, so it's definitely going!! Contact him at email@example.com for more info.
The guys who run this are active in the beaches league and are setting up some tournaments and leagues they say will be less expensive than some of the others (as of fall 2009). Check their website or call Mike at (904) 514-4808
Jax Sport and Social Club (www.jaxsport.com)
Private organization sponsoring sports and social activities, allows people to sign up as an individual for a sport if they don't already have a team. They are actively looking for 4 on 4 teams as of July 2009, they have both 4 on 4 and 7 on 7, so check them out.
Jacksonville Scene (www.jacksonvillescene.com)
Fairly new private organization (started in '05 or '06) offering adult sports leagues and social opportunities.
Jacksonville Beach Parks and Recreation
They don't have any info on the website, but the Parks and Rec phone # is (904) 247-6236 - they offer several adult athletic leagues. Otherwise the website is Jacksonville Beach (www.jacksonvillebeach.org)
City of Jacksonville Parks and Recreation (www.coj.net/Departments/Parks+and+Recreation/Recreation+Activities/Athletics/Adult+Athletics.htm)
The City of Jacksonville runs quite a few adult leagues as well. Their website is often not updated either, but you can call for details on upcoming leagues and league rules
Adult Athletics Supervisor