On the Development Seed blog today there's a new post by Alex Barth about integrating OpenID support into your Drupal installation. You can check out an example of it in this github project.

After a survey of available single sign-on solutions [for a client], we decided to go with an OpenID based approach since we needed to support different domains, wanted to avoid sharing user tables and did not want to add complex system requirements for browser clients or the server. [...] The great advantage of this scenario is that we know which five sites need to play nicely together and all of them are Drupal sites under the client's control. This premise allows us to add an additional site as a designated OpenID provider that we call "Hub" and make all five sites point to the Hub as their default identity provider.

They use two modules to make everything work together - OpenID SSO and OpenID Provider SSO and a PubSubHubbub model for keeping the user information up to date.

On the Lullabot.com blog there's a recent post from Nate Haug showing how you can set up a single sign on with Drupal even if your user crosses multiple sub-domains on your site. The technique is particularly handy in that it doesn't require any extra modules to be installed to make it work.

With the multitude of single sign-on modules out there for Drupal, it's easy to miss the fact that Drupal has a built-in single sign on mechanism already. No modules, no configuration, just 20 easy lines of PHP in your site's settings.php file. This solution works for a lot of clients, but the set of requirements is pretty specific as to when you can use this approach.

To make it work all sites must be on the same domain (just sub-domains of it), be using MySQL as your backend database and, if you're using clustered hardware, they need to be on the same cluster to be able to make the cross-database queries. Since Drupal can prefix tables so the settings for each site are split out, you can create a "shared table" by making a few changes in your master and slave configuration.

Arnold Daniels has a new post to his blog today dealing with something (usually companies) are looking towards to help deal with the infamous "too many passwords for too many places" issue - a simple single sign-on tool that can be dropped in an used anywhere.

Associated websites often share user information, so a visitor only has to register once and can use that username and password for all sites. A good example for this is Google. [...] There are many single sign-on applications and protocols. Most of these are fairly complex. [...] I've written a simple single sign-on solution (400 lines of code), which works by linking sessions. This solutions works for normal websites as well as AJAX sites.

He compares the flow on a non-single sign-on site (lots of fetching between the client/server) and the first/following visits with his tool in place. You can download the source here (as well as the Ajax broker).