• LEO is the online dictionary for native speakers of German. It has English, French, Mandarin Chinese, Italian, Spanish and Russian. I use it for the first three. LEO is very basic, it just gives you a list of all translations for a word or phrase with no ranking and very little information about usage and context. But the lists are often quite complete, it’s fast and there is also a message board that sometimes has useful additional translations and usage information. Here, as with everything on the Internet, using one’s brain while looking things up is important (as opposed to trusting the next best piece of information blindly).
• Recently, I have frequently found English words on dict.cc which were missing from LEO.
• The English Wiktionary has vast amounts of information on Chinese characters and their usage in Mandarin Chinese.
• A third source for Chinese is Arch Chinese, which I use mainly for stroke orders, and sometimes for decomposition into radicals. The latter is done great here in principle, but alas, it’s quite incomplete.
• Linguee is a new and hot thing with clever use of language technology. You can search English/German parallel corpora and see how others have solved the problem of translating XYZ. Now also available for English/Spanish, English/French and English/Portuguese.
• If you already have an idea of how to put something and want to check it against text written by others,  or when you just want to get an idea of how a word is used, Google is a classic of course. With some (especially rare) words, the 1,000,000,000 dictionary sites out there tend to get in the way though and clutter up the first few results pages – when what you want is precisely not a dictionary, but unedited real-life usage. For that purpose, I have defined the Serchilo command “Google as a concordancer”, which filters out all dictionary sites I have discovered so far. Even if you are not yet using Serchilo as your default “search engine” (which you should), you can use the command by typing serchilo.net/cong yourword in your browser’s address bar.
• As a grammar geek, I love the Logos Universal Conjugator, a vast archive of verb inflection tables in many languages. I mainly use it for French and Latin, when I need a more outlandish form of an irregular verb (or of a regular verb, for that matter) that I never bothered to remember thoroughly.

Dear readers, what are your favorite online tools for mastering foreign tongues?