Actually the best online Hebrew-English and English-Hebrew dictionary is Morfix
(way better than Google Translate, although Google Translate will sometimes give you a better translation of common short phrases). With Morfix you can type in any form of a word with any prefixes and/or suffixes that it could possibly occur with and it will give you every possible dictionary entry for that word, with proper vowels and good definitions. For example, typing in משחק
will bring up the entries for mischak (= game), sichek (= to play; because of the form mesachek), shachak (= clouds; because of the orefixed form mi-shachak), and even chek, chok, and chak (because each of those can be prefixed with mi-she-).
I generally use this for all purposes (whether Modern, Mishnaic, or Biblical Hebrew, although it is certainly geared towards Modern), but have a copy of Even Shoshan, probably the best existing comprehensive Hebrew-Hebrew dictionary, for when I need more detail. I often find myself reading a definition from Even Shoshan and looking up the unfamiliar words from the definition in Morfix!