1. Multitran: an excellent resource, Multitran has dictionaries for French, German, Spanish, Italian, Dutch, Estonian, Latvian, Japanese and, oddly, Afrikaans, Esperanto and Kalmyk. You very occasionally get a duff translation, usually of an idiom, but the only real problem is the absence of stress marks, which my students find a bit annoying.
2. Slovopedia: with links to Dal’s Etymological Dictionary, dictionary of Russian synonyms, Brockhaus-Efron Encyclopedia, and a number of specialist dictionaries, what more could you want?
3. Russian Slang: a guide to the weird, wonderful and complex world of Russian slang, with some good explanations and examples.
4. Russian Word of the Day: not a dictionary, I know, but a brilliant site for word-nerds, dealing with all sorts of peculiarities and problems of the Russian language, and with an added bonus: pictures of kittens.