Dictionaries
Old English - Modern English Dictionaries
Dictionary of Old English Links to an external site., ed. Healey, A. diP. et al. Toronto: Centre for Medieval Studies, 1987-. [the best Old English dictionary available - goes to I]
An Anglo-Saxon Dictionary Links to an external site., ed.Bosworth, J. and J.N. Toller. Oxford: Oxford Univ. press, 1898. Supplement, Toller, 1921; Revised and Enlarged Addenda by A. Campbell, 1972. [the best full Old English dictionary, but old]
Clark Hall, John R. A Concise Anglo-Saxon Dictionary Links to an external site.. 4th ed. Cambridge: Cambridge Univ. Press, 1960. (also available here Links to an external site.). [shorter than Bosworth-Toller, a bit less cumbersome, often has good alternate definitions]
Dictionary of Old English Web Corpus Links to an external site. (Social Sciences and Humanities Research Council of Canada and National Endowment for the Humanities, 2009) [has every occurrence of every word, not just A-I, but listed by individual spelling, not in dictionary form]
Learning with the Thesaurus of Old English Links to an external site.at the University of Glasgow. This is not an alphabetized dictionary but a thesaurus organized around semantic groups (food, clothing, entertainment, etc.).
Modern English - Old English Dictionaries
Walter W. Skeat, An English-Anglo-Saxon Vocabulary Links to an external site. (Cambridge: Cambridge University Press, 1879) - old but excellent!
The Old English Translator Links to an external site. - translate from OE to Modern English, or vice versa!
Modern English - Old English Dictionary Links to an external site. - not bad!
EOW: Englisc Onstigende Wordbōc Links to an external site. - also not bad!
Freelang English-Old English dictionary Links to an external site. - not quite as good, but worth looking at
Circolwyrde Wordhord, Links to an external site. a glossary of computer terms translated into Old English. A delightful height of geekiness.
Other Dictionaries
Oxford English Dictionary Links to an external site. (and see their information page on Old English in the OED Links to an external site.)
Middle English Dictionary Links to an external site.
Etymonline Links to an external site. - excellent online dictionary of etymology
Logeion Links to an external site. - for Latin and Greek!