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).
CKAN powers a number of data catalogues on the Internet. The Data Hub is an openly editable open data catalogue, in the style of Wikipedia. The UK Government uses CKAN to run data.gov.uk, which currently lists 8,000 government datasets. Official public data from most European countries is listed in a CKAN catalogue at publicdata.eu. There is a comprehensive list of catalogues like these around the world at datacatalogs.org, which is itself powered by CKAN.