[opan]

IRC + Mumble is a good combo in my experience. I only fire up Mumble as-needed since I tend to prefer text chat.

[jagermo]

i agree, but you need your people to have 2 logins. Great for friend groups, not working in scale.

[opan]

Neither one strictly requires a login AFAIK, actually. You can chat with an unregistered nick on IRC, and Mumble has certs, but I think they're optional and don't have associated passwords.