the best league in the city is (hands down) urban professionals. although to be honest, city leagues are popular - but sometimes poorly run and ALWAYS expensive. you'll pay over $100 for a season, and sometimes won't even get a t-shirt. boo. but yeah, urban professionals is the best one. i know they just started a new season a week ago, so keep an eye on their website for when the open gym to find a team is. it won't be for about a month and a half or so.
here's the site:
Welcome to NYUPAL Volleyball
as far as open gyms, well... new york really isn't the best place for that. but if you feel like having some fun, check out the concrete (i know... ouch) courts in central park. they're on 84th-ish on the northeast corner of the great lawn, behind the basketball courts. there are 2 courts back there and on the weekends, you get a nice draw of people. watch out for the regulars though, they're kind of snobby and won't let you play if you come alone. they're pick up games, and they'll last from 9am until it gets dark.
have fun :-)