It is a common opinion that today's digital libraries (DL) can no longer be simple "expositions' of digital objects. Users should no more be passive readers, they need to interact with the library, add their annotations and tags, personalize their experience and collaborate with each other. Web 2.0 technologies, such as social bookmarking and online discussions, are already being applied in DLs to allow users to annotate digital objects. However, the lack of semantic structure of such

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... s and a clear social model to share and aggregate community contributions makes it difficult to take full advantage of such collaboratively created knowledge. The SemLib project aims at developing a modular and configurable annotation system that can be easily plugged into existing digital libraries in order to allow end-users as well as digital libraries content curators to produce meaningful and customizable aggregations of semantically structured annotations produced by communities. In this paper we introduce the SemLib project, discussing the principles and ideas behind the proposed annotation system, and present a prototypal implementation.