On Tuesday, Single User Login became available for all users in an opt-in basis. This software feature (known as the CentralAuth extension to the software) will initially allow a user to reserve an account on all public Wikimedia wikis simultaneously; that is, nobody can create an account with the same username as you, and you can log onto any wiki without having to explicitly create an account there. It also means that the accounts are identified within the software as belonging to the same user, and synchronises their email address and passwords; also, logging onto one wiki logs a user onto all other wikis within the same domain (for instance, logging onto the English Wikipedia logs a user with a global account onto other language Wikipedias too, and logging onto Meta also logs a user onto Commons, because they are both at wikimedia.org; this feature was only enabled recently, so you may need to log out and back in again to benefit from this). Note that administrators were able to opt-in to single user login in March, and many have already done so; there is nothing special about single user login accounts created during the pilot, and administrators who already have such an account need not take any special action.
In order to opt-in to single user login, you need to create a global account. The simplest and most common case is where all your accounts have the same username, and nobody else has the same username as you. In this case:
However, there are several possible complicating factors:
If something goes wrong:
At present, single user login accounts have no effects other than synchronising email/passwords, preventing other users creating accounts with the same name, simultaneous login within a domain, and allowing their owner to log in to wikis they've never visited without explicitly creating an account; for instance, it will still be possible to have different preferences on different wikis. Note that at present, a single user login account cannot be renamed. Also, note that SUL is currently completely opt-in; you can just ignore it for the time being if you want.
There are several feature requests that single user login may make possible in the future; in particular, developers are currently looking at improving the login system so that it logs people in and out of all public Wikimedia wikis simultaneously (including those on other domains). Some examples of suggestions for other potential uses of the feature that have been made (which may be the basis of future development) include a new-messages bar, the ability to upload images to Commons directly from other wikis, and cross-wiki watchlists.
Another feature added this week allows "global groups", groups on the Meta-Wiki that give certain users rights on all public Wikimedia projects. Users must be "global users" through the SUL process in order to become a member of a global group. So far, only stewards have rights through the system, allowing them to use most administrative or other rights without explicitly giving themselves the rights each time.
Under discussion is the addition of various other groups, including a group for authorized Wikimedia employees. Global access to privileges like the "rollback" privilege, as well as the ability to move pages across all wikis for trusted users, particularly those who patrol smaller wikis for vandalism, has also been discussed.