The Data Hub is a community-run catalogue of useful sets of data on the Internet. You can collect links here to data from around the web for yourself and others to use, or search for data that others have collected. Depending on the type of data (and its conditions of use), the Data Hub may also be able to store a copy of the data or host it in a database, and provide some basic visualisation tools.
This site is running a powerful piece of open-source data cataloguing software called CKAN, written and maintained by the Open Knowledge Foundation. Each ‘dataset’ record on CKAN contains a description of the data and other useful information, such as what formats it is available in, who owns it and whether it is freely available, and what subject areas the data is about. Other users can improve or add to this information (CKAN keeps a fully versioned history).