DocumentsOnline (formerly PRO-Online)
"allows you online access to TNA's collection of digitised public
records, including both academic and genealogical sources".
National Register of Archives
is an extensive searchable database
(formerly run by the Royal Commision on Historical Manuscripts, now part of TNA)
which contains information on the nature and location of manuscripts and historical records
that relate to British history.
Your Archives - "The National Archives' online community of records users", a wiki that archive users can contribute to: "These pages are for you to contribute your knowledge of archival sources held by The National Archives and by other archives throughout the UK."
- Your Archives