Going to mention my own project which aims to be 100% open source, free, and relies almost only on public infrastructure: https://github.com/robertsdotpm/p2pd
Basically, I'm building a framework for building NAT traversal plugins. Software like ngrok and P2P VPNs can then be built on top of it. Examples of plugins for the library include direct connect, reverse connect (connect back to you), TCP hole punching, and UPnP-based port forwarding.
The underlying network stack for the project was also built from scratch to better support IPv6 and multiple interfaces. This allows plugins to fully utilise the underlying network paths and interfaces on the machine. This took considerable time because most software simply uses the default interface.
I'm still in the middle of building the software so its not yet functional. But if anyone is interested throw me a star or an email at matthew@roberts.pm.
By the way I forgot to add: if anyone needs a list of public STUN, TURN, MQTT, or NTP servers I wrote a monitor for them last year and added a bunch of servers. This is basically the infrastructure I use for my P2P library. The public API is here: http://ovh1.p2pd.net:8000/servers or if you want to host it: https://github.com/robertsdotpm/dogdorm