Socrata – A variety of data sets with a social vibe.
Timetric – Timetric aggregates statistics from the the world’s leading sources of economic data
Google Public Data Explorer – Anyone can upload data to Google’s newly launched search service.
Infochimps – Find every dataset in the world. Upload datasets and point to others across the internet. Datasets are easily browsed and the metadata is contributed by users.
Datamarket – UN, World Bank, Eurostat, Gapminder and others all contribute to 13,000 dataests (both paid and free).
Data.Gov – American government data
Data.Gov.Uk – UK government data
Get The Data – A forum of data geeks helping you with your data queries.