Grammars and dictionaries
Biblical Hebrew: Gesenius dictionary, English translation of 1906. Grammar of Gesenius-Kautzsch-Cowley, as a module in the free Bible program E-Sword.
Postbiblical Hebrew and Aramaic: Jastrow, Dictionary...; Margolis, Manual of the Aramaic Language of the Babylonian Talmud.
Arabic: E.W. Lane, Arabic-English Lexicon.
Syriac: J. Payne Smith, Compendious Syriac Dictionary; download PDF of the same; W. Jennings, Lexicon to the Syriac New Testament (download PDF). Nöldeke's Grammar download as PDF.
Coptic: W.E. Crum, Coptic Dictionary.
Akkadian: Association Assyrophile de France, Akkadian Dictionary.
Hittite: Association Assyrophile de France, Hittite Lexicon.
Latin: W. Whitaker, Words.
Biblical Greek page of Zack Hubert, user-friendly interface at Tyndale (choose Greek (OT & NT)).
Many modern languages: The Free Dictionary, French, Spanish, Russian etc.