Where does most of the ethereum developers hang out? Slack? Telegram? IRC? I'm looking to join a chatroom where everyone can help others while developing.
Gitter is a great place to start. You can discuss specific technologies with devs on various projects:
To check out what the core devs are working on, check out their Gitter channel.
For more long-winded questions and responses, check out /r/ethdev.
Twitter is also an excellent resource for direct interaction with people.
I have found that devs hang out on project-specific subreddits (0x, BAT, etc.), so asking a targeted question there should yield an answer.