These are some Internet sites that I've found useful in my research. I'd appreciate hearing about any quality resources that are not on this list!
Dictionary of the City of London
A gazeteer of over 6,000 street and place names in London; their location, origin and changes. Medieval references are frequent. An invaluable reference work.
Europaische Stammtafeln Notes
The Europäische Stammtafeln is a collection of genealogical tables for important families that played a role in continental European history. A complete index to all volumes of ES is found at Stammtafeln Register.
Foundation for Medieval Genealogy
Objectives: to promote rigorous scholarship in the field of medieval genealogical, prosopographical, and related research. Membership is required to view some of the resources; however, of particular value are the publicly-available corrections to Keats-Rohan's two-volum prosopographical series, Domesday People and Domesday Descendants.
The aim of GEN UKI is to serve as a "virtual reference library" of genealogical information that is of particular relevance to the UK & Ireland.
The Monumental Brass Society
Photographs and illustrations of medieval monumental brasses. Another excellent site with images of brasses can be found at The Gothic Eye.
The Herald's Visitations
Linked from Nigel Batty-Smith's Website, United Kingdom Genealogy. Surveys online include Cornwall, Devon, Somerset, Wilts, Worcester, Hertford, London, Norfolk, Kent & Berks.
Internet Medieval Sourcebook
History, music, literature and genealogy of the medieval era across continents, from Fordham University.
From Georgetown University's Medieval Studies Program. Easy-to-use menus and links provide connections to databases, services, texts, and images on other servers around the world.
Latin Genealogical Word List
This general word list includes words commonly seen in genealogical sources. Numbers, months, and days of the week are listed both here and in separate sections that follow this list.
Latin Translator ["Words by William Whitaker"]
Short phrases can be translated by this limited program.
Libro: The Library of Iberian Resources Online
Researchers will find a study of Iberian resources of interest because of the region's connections to the Plantageney dynasty. Emphasis on peninsular history from the 5th to the 17th centuries.
Medieval Names Archive
Etymology of English, Old English and Anglo-Norman names.
A pathfinder for medieval resources. Includes a searchable glossary of medieval terms.
Old English Links
A finding aide for everything from abbreviations to paleography.
The Online Reference Book for Medieval Studies
From Berkeley, written and maintained by medieval scholars for the benefit of their fellow instructors and serious students.
Some Corrections and Additions to The Complete Peerage
A link from the below listed site, "Some Notes," etc. Discusses additional corrections to CP since the publication of the 14th volume, "Addenda and Corrigenda," in 1998.
Some Notes on Medieval English Genealogy
Extensive online compendium authored by Chris Phillips. Includes published records, medieval source material, monumental brasses, medieval English families on the Internet, and a "What's New" section.