FreeBMD contains the indexes to the civil registrations of Births, Marriages and Deaths in England and Wales from 1837. Coverage is excellent into the 20th century – more recent records are being added (transcriptions done by volunteers).
You can restrict a search to a particular registration district or county. If your search does not produce what you are looking for, try entering minimal information in the search boxes - perhaps just names and county and a broad time period, or just surnames without first names. The image above is an example of a search for a marriage of an Alice CURTIS in the Dec quarter of 1933 to the September quarter of 1934, without specifying any other information in the search.
After 1866 the age at death was recorded so this is useful in nailing down the birth year.
Unfortunately the indexes after about 1911 do not show middle names in full, just initials, so searching for someone with a middle name won't produce any hits for records after this period. In this case, you can still find middle initials, so for example, if you were looking for a death of a Robert John MacAdam, you could search for Robert J* MacAdam and that would find the Robert J's after 1911 and the Robert John's before 1911.
After 1912 the mother's maiden name was shown in the birth index, so you can use this when searching to find siblings of your ancestor by entering just the surname and the mother's maiden name.
If you think something may have been mistranscribed, you can view the original image of the index page by clicking on the eyeglasses icon.
You can search the same indexes that FreeBMD covers on ancestry.com - ancestry's coverage goes into 2005 or so (FreeBMD is only currently into the 1940s so ancestry is better for living people), but is not quite as flexible in its use of wildcards. After 1969, the death indexes include the date of birth - currently this is only available using ancestry.