OpenSim, the open source spinoff of Second Life, has a small but passionate userbase (probably between 10,000-15,000), many or most of whom are educators who use the technology as a low cost teaching tool. To enhance OpenSim's limited communication features for pedagogical events, educator Leighton Marjoram has turned to the popular gamer platform Discord:
As people who use Opensimulator will know voice and chat range whilst in world is limited and is not helpful when exploring regions together in groups. Discord and the overlay helps a great deal as all the grid crawlers have to do is signup for Discord and download the desktop client and join a dedicated channel (voice and chat)... Also while on the Opensim Life grid we decided to do a group build region and we used to Discord for a private discussion area and used voice in-world whilst we were building.
More here, along with tips for setting Discord up. For users who need a more secure and private chat channel, such as therapists who use OpenSim for RL therapy -- the US miliary, for instance, uses OpenSim as part of its therapy for personnel with PTSD -- Leighton recommends Matrix and Riot app.
Hat tip: Rocky Constantine.