Middle English Dictionary
The Middle English Dictionary is a dictionary of Middle English published by the University of Michigan. "Its 15,000 pages offer a comprehensive analysis of lexicon and usage for the period 1100-1500, based on the analysis of a collection of over three million citation slips, the largest collection of this kind available." 
The project began in the 1920s. The first instalment, "Plan and Bibliography", containing a list of Middle English texts used for the Middle English Dictionary, was published by Hans Kurath and Sherman Kuhn in 1954. More fascicles were published in numerous volumes (in alphabetical order) over the next several decades. The MED was completed in 2001.
In 2007 the full dictionary was made freely available and searchable online in an HTML format. The home page for searching the dictionary is available here.