One of our clients had requested an online file repository, mainly for secure document storage and distribution.  There are many ways to accomplish this but we decided to go with the Knowledge Tree engine.  It is opensource and there is a paid version or free version, with the paid version providing additional features and support over the free version.   With this application it is very easy to store and collaborate with documents online with many users only using a web browser to access and manage the entire portal.  There is a comprehensive user and rights management system in place to ensure only authorized users will have access to your documents.   Additionally you can install plugins that enhance the core functionality.   Two of the most notible features are the drag-drop upload feature which allows simplified file uploads to the portal, and the ability to email files from the portal itself rather than having to download and use your local email client.
 
Overall we are quite happy with the implementation and our client is happy as well.   We will definitely be implementing this product wherever there is a client need.