There's a particular discovery problem for open and distributed protocols such as OpenID, OAuth, Portable Contacts, Activity Streams, and OpenSocial. It seems like a trivial problem, but it's one of the stumbling blocks that slows mass adoption. We need to fix it. So first, I'm going to name it:
The Personal Web Discovery Problem: Given a person, how do I find out what services that person uses?
This does sound trivial, doesn't it? And it is easy as long as you're service-centric; if you're building on top of social network X, there is no discovery problem, or at least only a trivial one that can be solved with proprietary APIs. But what if you want to build on top of X,Y, and Z? Well, you write code to make the user log in to each one so you can call those proprietary APIs... which means the user has to tell you their identity (and probably password) on each one... and the user has already clicked the Back button because this is complicated and annoying.
XAuth is a lot like democracy: The worst form of user identity prefs, except for all those others that have been tried (apologies to Churchill). I've just read Eran's rather overblown "XAuth - a Terrible, Horrible, No Good, Very Bad Idea", and I see that the same objections are being tossed around; I'm going to rebut them here to save time in the future.
Let's take this from the top. XAuth is a proposal to let browsers remember that sites have registered themselves as a user's identity provider and let other sites know if the user has a session at that site. In other words, it has the same information as proprietary solutions that already exist, except that it works across multiple identity providers. It means that when you go to a new website, it doesn't have to ask you what your preferred services are, it can just look them up. Note that this only tells the site that you have an account with Google or Yahoo or Facebook or Twitter, not what the…
Twister is interesting. It's a decentralized "microblogging" system based on putting together existing protocols: Bitcoin, distributed hash tables, and Bittorrent. The most interesting part for me is using Bitcoin for user registration and spam control. Federated systems handle this with federated trust, which is at least conceptually simple. The Twister/Bitcoin mechanism looks intriguing though I don't know enough about Bitcoin to really comment. Need to read further.