Here's my list of favorite websites that help me daily in my work as a French-English translator. Some of these online tools extend to other languages, and some are specifically geared towards French and English. In any case, I hope that you will find some of these useful!
Le Grand Dictionnaire Terminologique (GDT): A wonderful French ↔ English technical dictionary for many (if not all) scientific fields. Launched and updated by the Quebec government, it's very complete and reliable, since it differentiates the uses of a word according to the specific scientific field.
Linguee: Enter a word in your source language (say - French), and you end up with the word in your target language (English), used in full sentences taken from available online documents. This tool is a hybrid between a bilingual dictionnary and a search engine. It's mainly for French ↔ English, but there are beta versions in French ↔ German, Spanish and Portuguese. I would qualify Linguee as being very user-friendly and generally accurate. But, one must - of course - remain vigilant when looking at the search results, since the translations on offer can vary in quality and precision. I usually use Linguee as the starting point in m y search for the right word, to see different examples of the word in context.
ProZ word discussion forum: ProZ is an online platform for professional translators of all languages, and it offers many services that have to do with terminology. The glossary allows you to enter a word or phrase in the source language and then gives you multiple options in your target language, gleaned from proZ terminology forums.
There's also the live discussion forum on specific words and phrases, called KudoZ. You can ask a terminology question and get answers from other translators as well as provide answers if you are familiar with the term in context. I use the Proz tools especially when I need to translate an unfamiliar full expression, since many expressions are referenced and have been discussed by translators.
If you know of any other very useful tools, don't hesitate to tell me all about them! I'm always on the look out for reliable online dictionaries.