Wikimedia Downloads: Database tables as sql.gz and content as XML files

See the list of database backup dumps.
Also see daily dumps of additions/changes to content (Experimental!)
Historical material only: archives of sql/XML dumps for previous years starting from 2001

Other content

Pagecount data collected by Domas Mituzas
Pagecount/projectcount data derived by Erik Zachte from Domas Mituzas' archives
WMF Survey data
Picture of the Year Zip or GZ files
The top 6 submissions of the WikiChallenge data competition, contains source and data
April 2011 English language Wikipedia revisions as additions/removals to the previous text
1911 Edition of the Encyclopedia Britannica, scanned as tiff files
User signup data for the Wikipedia Education Program