Synchronization system and associated methods provide synchronization of an arbitrary number of datasets, including more than two datasets. To achieve this, a reference dataset is used to store a super-set of the latest or most-recent data from all user datasets to provide a repository of information that is available at all times. Therefore, if the user later wishes to synchronize a new user dataset, such as one in a server computer that stores user information, the system already has all the information necessary for synchronizing the new dataset, regardless of whether any of the other datasets are then available. Further, to simplify use, a unified user interface is provided that allows the user to easily determine which of his or her datasets are currently set to be synchronized and allows the user to conveniently alter the current settings to select one, two, or even more than two clients for synchronization. Various “conflict” or “duplicate” resolution strategies are described for intelligently handling complexities resulting from allowing synchronization for an arbitrary number of datasets and allowing synchronization using even data from datasets that are not available. Architectural support for “plug-in” client accessors and type modules is also provided. This allows support to be added for new datasets or new types of data merely by developing and plugging in new, compact client accessors or type modules, without updating or replacing the core synchronization engine.