Tumult is a totally anonymous, end-to-end encrypted random chat service. Users are given a random identifier, and are thrown into a chatroom with up to 20 other people. The rooms themselves are also anonymous, and there is no way to join a specific room. You are thrown into a random room, and you can choose to stay, change to a new random room, or you can change your random identifier.
Users can be banned from a room by vote, though this only lasts as long as those who voted stay in the room. If those people leave and all new people take their place, the ban is lifted. Of course, rejoining that specific room will only happen by random chance, so if you get voted out and want to return later... good luck!