Access to the electronic Middle English Dictionary is currently free on a trial basis.
The print MED, completed in 2001, has been described as “the greatest achievement in medieval scholarship in America.” 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. This electronic version of the MED preserves all the details of the print MED, but goes far beyond this, by converting its contents into an enormous database, searchable in ways impossible within any print dictionary.
Grab it while you can! (Thanks, as so often, to aldiboronti at Wordorigins.)