AnkiStrictly-speaking this is 'Spaced-Repetition Software'. It's ideal for making your own virtual flashcards, but its main strength is that the application uses some extremely sophisticated logic to control how often you see each card, depending on how easily you remembered it last time around. Lots of clever extras, very easy to use, and free.
AntimoonAntimoon is aimed at learners of English as a 2nd language, but the site contains lots of useful myth-busting facts and articles about how to learn any foreign language. It also contains sections on the value of Spaced-Repetition Software, and on the necessity of receiving lots of 'input' in your chosen foreign language.
ForvoLots of people of different nationalities have contributed recordings of words and phrases in their native language. Just start typing the first few letters of a word in any language, pick the full word from a list of possibilities, then choose which recordings to listen to.
Google TranslatePick your 'To' and 'From' languages, type in a word or phrase, and see it instantly translated. Then click the loudspeaker icon to hear the translation spoken aloud. This is an invaluable tool for every serious language learner.
SuperMemoSuperMemo was one of the earliest spaced-repetition software programs. As a result, this site contains lots of valuable information about the science of memory & learning, with hints, tips, principles and scholarly articles. Very useful if you're interested in the theory of how and why Spaced-Repetition Learning works.