Below are a few of the links that I have found useful in my genealogical research.

Online databases

FreeBMD   Civil registration index entries for England and Wales (births, marriages and deaths from 1837-1901).
FamilySearch LDS records including the International Genealogical Index. To find the IGI batch numbers for a specific town or parish visit this site.
CWGC Commonwealth War Graves Commission website, with names of all members of the Commonwealth forces who were killed in the two World Wars.
JRI Poland Indexes to 19th century Jewish vital records from current and former territories of Poland.

Maps

Old maps   Extensive digital archive or historical Ordnance Survey maps.
Multimap Street-level maps of the UK, detailed maps of Europe and overview maps of the rest of the world.

Yorkshire genealogy

GENUKI Yorkshire   GENUKI's pages of information specific to the county of Yorkshire.
Topographical Dictionary Information about Yorkshire towns, villages and hamlets, alphabetically arranged under the headings of the North, East and West Ridings.
Wath and Dearne Valley Resources Parish register transcripts, trade directory indexes, historical photos and more, covering the Wath-upon-Dearne area of Yorkshire.