http://slack.bittrex.com

http://www.whaleclub.org/videos-hangouts/

https://thedao.slack.com/app (Some of the ether devs hang in here too)

https://slack.bitcoincore.org

I also hang out in the EXP, blocknet, and XSt slacks. EXP being the most active by far. Must of been mistaken on a bitfinex and polo slack channel. But most these communities are active traders.