1821 Census - Fire destroyed a lot of archived census material in 1922, so info is limited. 1831-1851 is also available on Family Search with further reduced info.
1901 & 1911 Irish Censuses - Searchable facility
Belfast Cemetery - Burial records from 1869. City Council. Searchable facility.
Births and Baptisms - 1620-1881 [Family Search].
Cemetery Records - Search by county.
Cemetery Records - Alpha lists.
Civil Registration Indexes - Family Search.
Convicts - 1788-1849 transported to New South Wales.
Cork - 1845, Directory. Alpha list. In .pdf format.
Dublin - 1850, Directory. Alpha list.
GENUKI - Ireland, recommended reading, very informative.
Griffiths Valuation -1847-1864. Based property valuations, excellent source of info [lack of censuses]. Searchable database.
Irish Genealogy - Various searchable indexes.
Marriages - 1619-1898, Family Search, searchable facility.
National Library of Ireland - Well worth a look.
Petty Sessions Court Registers - 1828-1912 [Family Search]
Prison Records - 1790-1924 [Family Search].
PRONI - Public records of Northern Ireland. Various databases.
RootsIreland - Searchable database.
Tithe Applotment - 1814-1855. RoI [Family search]. Searchable.
Tithe Applotment - 1823-1837. National Archives of Ireland. Searchable database.
Wills - 1858-1929 [Family Search]
Wills and Administration - 1858-1922 [National Archives of Ireland] searchable.
Ayrshire Roots - Not checked yet, but I understand it is a free to use database.
Dundee - FDCA. 1801 census. A nice find, well worth a look at the full site for other local snippets.
Gazette, The - Search the archives.
Gravestones - Photographic resource.
Immigrant Ships - Depearting from Scottish ports. Includes ships and some passenger lists.
Perth Burgh - Burial registers, searchable database, 1794-1855
Poorhouses - Summarised lists which includes the 1881 census in most cases.
Post-Office Directories - Online books or download as a .pdf
Scotland Births - [Family Search] Maybe either/or Births/Baptisms, 1564 - 1950
Scotland Marriages - [Family Search] 1561-1910
Scotlands People - Essentially a PAY SITE, but you can search the indexes for free and it will say if there are matches. There are a few free facilities.
Scottish Indexes - (Graham Maxwell Ancestry) 1841 - 1861 censuses and a number of other searchable indexes.
SCTBDM - Scotland BDM site, read the page this is linked to before going to 'Site Search'
Shetland Family History - Searchable facility and more.
Tiree - Old parish registers.
Wigtownshire - Old Parish Records.
Wills - 1513-1925, 'soldiers wills' also on foot of same page.
Births/Baptisms - [Family Search] Wales, 1541-1907. Read the description first.
Caerleon - Newport, South Wales. Nice, concise site with searchable/listed Parish Records. Well worth a look at the full site.
Conway - Parish Register, Diocese of Bognor, 1541-1793. Downloadable or just search the book.
Crime - Crime & Punishment. Searchable database 1730-1830
Glamorgan - Parish registers 1558-1900
General Register Office - [GRO]. Search birth and death indexes [1837-1916] for mother's maiden name [bitrths] and ages [deaths]. You will need to subscribe [free].
Llangar - Parish Records, 1640-1831.
Llansannan - Parish Register, 1667-1812. Downloadable or just search the book.
Marriages - [Family Search] Wales, 1541-1900. Read the descriptin first.
Monmouthshire- Various transcribed Parish Records, well worth a visit if your relatives are there.
Newport - South Wales. St Mary's Church. Searchable Baps, Marriages and Burials, various dates to 1915.
Swansea Mariners - Mariners from Swansea and those who sailed from Swansea. Large searchable database, includes name, year and PoB and ship.
Welsh Wills - Proved in the ecclesiastical courts before 1858, searchable database. In Welsh.