Recommendations for a good app/ website/ tech to create an informal message board/ server type thing to communicate with the other faculty at my school

I’m trying to come up with a less official channel than school wide, reply all emails where we the teachers could discuss less pressing stuff like advertising goods and services or just for asking around about whatever. I’m not trying to make a back channel for talking ish about the administrators but more a place to promote things we can’t when at work discussing official business. I’m guessing something like this already exists somewhere already but I don’t know what it is…