Vote: Linguee
Why: besides storing definitions individually submitted and approved as a regular dictionary, it crawls the web (or specific sites, maybe) for existing bilingual/translated texts and shows these occurrences side by side when you search for a word. This contextual knowledge is invaluable.
It is also bidirectional, i.e., you don't have to say if it's from English to Portuguese or backwards. (Simply select the pair and it's done.)
Tip: using the Add to Search Bar (restartless) addon on Firefox, you can create custom search engines for any language pair. I have done it with several pair, giving each of them a keyword:lenpt stands for Linguee English-Português, lenes for Linguee English-Español, lespt for Linguee Español-Português, etc. So I just open a new tab and type lenpt installation and Firefox automatically opens the page seen in the screenshot above.
This tip also works for Google Translate: is use gten as a keyword to translate (from an automatically detected source language) to English, gtfr to French, and so on. The search engines for each target language are available with a simple search at Mycroft. (There are some Linguee ones there too, but very few.)
I use both Google Translate and Linguee, since they complement each other.
(Another trick that sometimes helps is using Wikipedia's sidebar with links to the same article in other languages, but I wouldn't qualify this as a proper "translation tool"...)
