ABBYY Lingvo is the best out there. I don't know about the online version, but the computer software is hands down THE BEST. First off, it's the most comprehensive one. Way better than google translate or Babylon. It finds the definition to the weirdest slang words, poetic words, and hardly used words. Secondly, It has inflections for every word (the cases for nouns, conjugations for verbs), always shows you perfective and imperfective versions of verbs, so it's a huge help if you're writing in Russian. It also has the ability to click on a word so you won't have to write it down in the dictionary. The way it's built and designed is very smart, effecient and handy. So forget the online tool, go with ABBYY Lingvo.