OGSA-DAI (Data Access and Integration) is perhaps less well known and understood than some other Globus components. However, it is one of the most useful and successful. Developed in the UK, it provides uniform Web Services interfaces to diverse data resources. These interfaces allow clients not only to "access" data, but also to query, update, transform, and deliver it. In other words, they let you specify some pretty fancy server-side operations.
We've been using OGSA-DAI recently in the new GT4 GRAM audit service, with good results. Audit records generated during job execution are stored in a database and can subsequently be retrieved by (authorized) clients. OGSA-DAI is used to create a virtual database from internal audit and accounting databases. The experience has been positive and has emphasized to me the value of the OGSA-DAI abstractions and implementation.
For more details on OGSA-DAI, visit their Web page. Among other things, they have an inspiring list of projects. And I like the octopus. Funnily enough, I raised the idea of an octopus as a Globus mascot a few years ago, but no-one seemed to like the idea.