MIT’s Semantic Interoperability of Metadata and Information in unLike Environments (SIMILE) project is producing a variety of interesting open source software packages that will be of interest to librarians and others such as Piggy Bank, "a Firefox extension that turns your browser into a mashup platform, by allowing you to extract data from different web sites and mix them together."
Here is an overview of the SIMILE project from the About SIMILE page:
SIMILE is a joint project conducted by the MIT Libraries and MIT Computer Science and Artificial Intelligence Laboratory. SIMILE seeks to enhance inter-operability among digital assets, schemata/vocabularies/ontologies, metadata, and services. A key challenge is that the collections which must inter-operate are often distributed across individual, community, and institutional stores. We seek to be able to provide end-user services by drawing upon the assets, schemata/vocabularies/ontologies, and metadata held in such stores.
SIMILE will leverage and extend DSpace, enhancing its support for arbitrary schemata and metadata, primarily though the application of RDF and semantic web techniques. The project also aims to implement a digital asset dissemination architecture based upon web standards. The dissemination architecture will provide a mechanism to add useful "views" to a particular digital artifact (i.e. asset, schema, or metadata instance), and bind those views to consuming services.
You can get a more detailed overview of the project from the SIMILE grant proposal and from other project documents.
There is a SIMILE blog and a Wiki. There are also three mailing lists.