We describe a collaborative annotation system for the production of rich media e-learning contents. Our system exploits a wiki-like interface that allows cooperative users to enrich didactical multimedia lectures with additional information, such as captions, annotations and comments. Before delivering them to users, these additional contents are automatically adapted and integrated with video lectures based on user profiles and requests. This way, the additional contents collaboratively provided by users not only represent an effective alternative that students may follow to better understand the lecture, but also allow for a fine-grained customization of the didactic material. This is an important aspect which represents a step forward towards the Web(2.0)-ification of e-learning technologies. An experimental assessment shows the viability of our approach.