Das Deutsche Wörterbuch von Jacob und Wilhelm Grimm
Odd German word got you down? Not sure if it's some weird plural? Wondeful and free internet version.
Latin and Greek Dictionaries at Project Perseus
Direct link to dictionary look-up at Project Perseus.
Extremely useful (and Free) Latin dictionary program, available for most platforms.
Simultaneously look up terms in several early modern French dictionaries. For the sciences and arts, be sure to check Furetière as well--it's available as a free download on gallica.bnf.fr.
OED (Columbia Only)
Thesaurus Eruditionis der DFG-Projekte: CAMENA - Corpus Automatum Multiplex Electorum Neolatinitatis Auctorum
Set of important early-modern reference works (in Latin): names, vocabulary, history, heraldry, genealogy, medicine, rhetoric, canon and civil law, fabulae, magic, Jesuit ratio studiorum, etc. Strong on Germans and Jesuits. Beautiful scans.
Orbis Latinus (Latin Place Names) Online (Columbia) and Latin Place Names (BYU)