Yggdrasil (200::/7) is a friend-to-friend meshnet with end-to-end encryption that carries mostly IPv6 traffic.
Think global IPSec - but it actually works.
It is kindof like the ordinary internets, except that it is "secure" by default. Secure, as in that all packets are encrypted end-to-end and authenticated. MITM-attacks are not possible: The nodes IPv6-addresses are the hashes of their public keys, so it is essentially impossible to impersonate any other node (not physically impossible, but quite impossible, depending on your trust in the science of modern cryptography). Routing is done using a novel DHT mechanism based on oldskool Kademlia. This also means that Yggdrasil does not rely on central authorities for address assignment; bureaucrats have no say, it is decentralized. There is no politics involved in routing decisions (unlike intertubes, meh). You can use Yggdrasil to organize your home LANs servers, and globally for connecting to other sites.
Yggdrasil have a built in firewall that can be used to drop everything excepts packets belonging to sessions that you yourself initiate. This can be useful for clients, if you just wish to explore the net. This is not enabled by default though. Read more about it under "SessionFirewall" in the docs.
While Yggdrasil is is good for secure authenticated communication, it requires extra steps for secure authenicated "hidden" communication. In particular, if you wish to be anonymous, you should probably try to avoid simultanously peering via the clearnet and Tor/I2P, as that could very well reveal your identity. Note also that your node IPv6 address does not change, unless you recreate your cipher keys (simplest to do by just recreating the config file.)
The network only have maybe 180-210 nodes attached to it now. It is what you bring with you. Just the utility of having an ambient authentication mechanism and transparant encryption is rather nice though. The Yggdrasil community is perhaps mostly made up of misfit sysop hackers.
An entry-point is provided by cryptoanarchy.org: Peer details.
(Only using Yggdrasil for your own servers is also possible of course. That way all your servers could be on the same virtual network, with encryption and authentication, automatically and with almost no configuration. Again see the docs about the yggdrasil firewall - it can be made to only accept connections from directly peered nodes: AllowFromDirect)
If you peer with this node, please know this: This node have limited bandwidth, which is also shared with a Tor exit-node. If you want good anchors in Yggdrasil, you need to find more peers and build tunnels of your own. It is probably most healthy when its links are friend-to-friend.
Other known entry-points: Git repo.
This map of The Yggdrasil does not show the hidden pathways - it only shows the branches used for navigating the labyrinth in O(log(n))-time. (?)
A list of public services. Some of them are only accessible from the inside.