Discord is an instant messaging platform that provides functionality similar to Slack and IRC, but also includes voice chat.
The LessWrong Discord server is located at https://discord.gg/eYqZMhm (easier-to-remember redirect: memeware.org/rationaldisco).
More details can be found in the LW post here.
There is also another Discord server for Slate Star Codex here, which has channels for such topics as 'Culture war' (a place to discuss blue-vs-green issues), 'Media', and 'Gaming' (cf. gamers).