1. A system that implements a service for assisting users in locating reference materials, comprising: an electronic data repository that stores content clippings and associated citations and metadata tags, said content clippings uploaded to the electronic data repository by clipping modules that run on user computing devices of a plurality of users of the service, said citations identifying source references from which the content clippings were obtained, said metadata tags including text that is descriptive of the corresponding content clippings; and a computing system comprising one or more computing devices, said computing system programmed to analyze text of a document of a user, and to select, based on the analyzed text of the document and based on the metadata tags and citations associated with uploaded content clippings, one or more source references to recommend to the user; wherein the computing system is additionally configured to use social network connections between the users to implement a content clipping sharing policy in which users selectively share uploaded content clippings with other users.
1. A system that implements a service for assisting users in locating reference materials, comprising: an electronic data repository that stores content clippings and associated citations and metadata tags, said content clippings uploaded to the electronic data repository by clipping modules that run on user computing devices of a plurality of users of the service, said citations identifying source references from which the content clippings were obtained, said metadata tags including text that is descriptive of the corresponding content clippings; and a computing system comprising one or more computing devices, said computing system programmed to analyze text of a document of a user, and to select, based on the analyzed text of the document and based on the metadata tags and citations associated with uploaded content clippings, one or more source references to recommend to the user; wherein the computing system is additionally configured to use social network connections between the users to implement a content clipping sharing policy in which users selectively share uploaded content clippings with other users. 2. The system of claim 1 , wherein the computing system is programmed to select the one or more source references based additionally of metadata tags of content clippings included in the document.