Although the Freedom of Information Act is good, journalists need to know what to FOI. The other side of this website would be people who work in government suggesting what to FOI.
All requests and suggestions would be tagged with themes and organisation names, making the site searchable, and allowing automated email to people interested. People could submit the organisation the work for, for example, and receive an update on all journalist requests for information.
The website should also provide guidance on how to leak (e.g. remailers, web anonymisers, etc) and the risks (e.g. info in the Official Secrets Act).
Some of these ideas are being implemented at Wikileaks.