Yggdrasil is a fun one - I used to have it installed on all my boxes and would use it to ssh into everything. It saved my bacon a few times when internet connectivity for one device went out but because it meshed locally with other devices on the lan that still had internet, traffic got rerouted automatically. LAN peering uses broadcast discovery using ipv6 link local addresses, so it works even if the router dies and you don't have dhcp handing out addresses.

I just set up a couple instances in the cloud and put a list of their connection details along with a few publicly run nodes as backups into the configs of each. As long as enough stays up for a path to exist you can reach it.