Available online at Ancestry, findmypast or The Genealogist and other collaborating web sites - index and enumerator book images.
Enumerator book images are available on CD (for Hertfordshire Unions) from British Data Archive. See Quality of Census Images - which compares CD images with the images available on Ancestry.
Indexed Transcript are available on CD from Hertfordshire Family History Society. (easy to use).
Transcript available on CD from Hertfordshire Publications. (Unindexed and the format and file structure format makes it difficult to search.) Some of the records are printed in the following books:
Book: Population ... Volume 1: The Berkhamsted Region - includes a transcript of the 1851 census for Berkhamsted, Northchurch, Tring (including the hamlets of Wilstone and Long Marston) and the villages of Little Gaddesden, Frithsden, Wigginton, Aldbury and Puttenham.
Book: Population ... Volume 2: The St Albans Region - includes a transcript of the 1851 census for Wheathampstead, Harpenden, Redbourn, Sandridge and St Albans, including the parishes of St Michaels, St Peters and St Stephen and associated rural areas.
As a pilot for the 1881 census, the LDS indexed the 1851 census for Devon, Norfolk and Warwickshire, and they have issued the data as a CD. While it does not cover Hertfordshire it can be searched for people born in Hertfordshire (as HRT) living in one of the indexed counties. However you should realise that the spelling of Hertfordshire place names is erratic and, for example, there were people living in Exeter, Devon from "Bishop Stafford", "Bishops Stortford", "Bps Stortford" and "Bishops Storiford."
Straw Plait Dealers 1851 gives an example of how the 1851 census can tie in with the 1851 trade directory.
If you can add to the information given above, including additional sources for 1851 census transcripts/indexes, please tell me.
Page updated Jamuary 2011