In the LoRA/radio device sense, Meshtastic[1] is probably the easiest to get started with. It's the biggest player in the space, has devices that come pre-installed and configured, the most likely chance of making contact with someone else, etc. MeshCore[2] is the other major player. It's newer and tends to have been adopted by communities that have run into issues with large Meshtastic networks.
If you meant PC-based mesh networking, I'll leave someone more knowledgeable to speak about that :).
If you meant PC-based mesh networking, I'll leave someone more knowledgeable to speak about that :).
[1] - https://meshtastic.org/
[2] - https://meshcore.co.uk/