Although many physical archives make some of their documents available online, working with their collections usually requires traveling to the archive. The Featherstone article cited on the Virtual Archives page is also relevant here.
National Archives (UK)—single access point for searches of records held by The National Archives and 2,500+ other archives in the U.K.
Archivportal—the Deutsche Digitale Bibliothek's comprehensive portal to archives across Germany
Wisconsin Historical Society Collections—exceptional strength in labor history and some large business collections