The Royal Historical Society Bibliography web site (http://www.rhs.ac.uk/bibl/) recently announced that it is converting from a free web site to a for-pay subscription site. The change will take place on January 1, 2010 so you have a lot of time left to use the free site.
The Royal Historical Society Bibliography web site contains an excellent database comprised of articles and books and extending back to the 1930s/1940s. It covers all periods of British and Irish history. For information, go to http://www.rhs.ac.uk/bibl/docs/news.html.
My thanks to Connie Reik for telling me about the changes.