Urbit is a fascinating computing project that is on the verge of transitioning from "early developer" to "early adopter" phase.
It is a breath of fresh air in the decentralization space, dealing well with the realities of networks and identity consensus without the use of a blockchain.
Unfortunately the description in the README is not compelling to a general audience:
Urbit is a clean-slate system software stack defined as a deterministic computer. An encrypted P2P network, %ames, runs on a functional operating system, Arvo, written in a strict, typed functional language, Hoon, which compiles itself to a combinator interpreter, Nock, whose spec gzips to 340 bytes.
What is this for? Most directly, Urbit is designed as a personal cloud server for self-hosted web apps. It also uses HTTP APIs to manage data stuck in traditional web applications.
More broadly, Urbit's network tackles identity and security problems which the Internet can't easily address. Programming for a deterministic single-level store is also a different experience from Unix programming, regardless of language.
An urbit is a personal server, a building block of digital sovereignty. Right now a dozen different companies control your data and you can only expose it by their rules. "Server app platforms" like Sandstorm or one-click personal server-side apps like Private Packets or Rocket Chat all have their heart in the right place, but Unix system administration sucks too much for this model to serve the general public, and the internet's public key infrastructure is still fundamentally insecure.
There is something to learn from urbit about every part of the OS/internet stack, but the most interesting aspect of Urbit to Steemians is likely to be the identity model. In short, identities are distributed hierarchally after which they are independent and just need to have neighbor nodes willing to route for them. An urbit name is best compared to a pronouncable IP address that is assigned deterministically, and looks like ~lagmel-dilfex. Each urbit records the key update histories of all urbits it cares to connect to. Read more about it here: http://urbit.org/posts/address-space/
The company building Urbit recently sold 4 galaxies' worth of stars. A star can issue about 65k planets. It is in the best interest of anyone owning chunks of the address space to give away enough of it that it finds some valuable users. So, here we are:
Planet Giveaway
Each account that upvotes or comments on this post in the next 7 days will be assigned a random planet from the star ~lorten (which has ~65k planets). Once the Urbit team has declared transfers to be future-proof, a distribution mechanism that uses your account's keys to authenticate (e.g. a message to you or a request for an input via comment) will be used to issue your planet to you.
Follow this account for updates. Note that if you want to start hacking right away can create a self-signed comet and the existing network will happily talk to you.
Of course, the disclaimer: This is a free giveaway and there are any number of ways your upvote could fail to turn into a planet. I will incentivize myself by not moving the SBD rewards for this post until the distribution is complete. If for some reason I can't deliver (like the Urbit team gets hit by a truck), I will send the rewards to 
If you have questions about Urbit I will do my best to answer them, although I am just a casual fan and not an Urbit dev. If this post gets popular enough I might suggest the Urbit team do another AMA on Steem.