matrix-voyager-bot
Voyager is a bot that travels the network by looking for rooms it can join. When the bot sees a message with a room alias, it will try to join the room to gather more links to rooms.
Check out the current graph, or ask questions in #voyager:t2l.io. Voyager is not meant to be an intrusive bot, and is more than happy to automatically de-list a room when kicked or banned.
The source (and more information) can be found on GitHub.