Below, you will find a list of the resources I use for my localisations and testing:

ACRONYMS
Acronym Finder: With more than 900,000 human-edited definitions, Acronym Finder is the world's largest and most comprehensive dictionary of acronyms, abbreviations, and initialisms.

Acronimia: It contains nearly 500.000 acronyms in 7 different languages, and it allows the user to add new ones, so it grows day after day.

Acronyms page of the Interinstitutional style guide: It is the official Style Guide of the EU, and you can find it in the 21 official languages of the EU Member States.

ONLINE FREE LANGUAGES DICTIONARIES
Wordreference: I have always a tab open with this dictionary. It might not be the most complete, but it is very easy to post a thread in the forum and most of the times you have someone replying instantly. Not everybody participating are experts translators, but sometimes you actually need "normal" people to help you find the most idiomatic translation for your word or sentence. It has many languages into English, but only French, English, Italian and Portuguese into Spanish.

Reverso diccionario: Another dictionary I tend to have always open when I work. It sometimes give me that perfect translation that I cannot find in Wordreference, but other times doesn't have the word I am looking for but I can find it in Wordreference, therefore, I think both dictionaries are a good partnership.

Pons.de: Probably the best German dictionary online. I bought a Pons dictionary (GER + SPA) when I was studying German and it was brilliant. But when I moved into the UK, I couldn't take it with me (it weighted about 2 kilos), so I had to find an alternative. So I found it! I have also seen that they have add many more languages (when I first used it, it had only German into English and German into Spanish), so maybe I should start using it also for my English translations. Worth a try!

Proz.com's terms: Proz.com is, probably, the most famous translators' workplace. There are translators from all over the world and they have a section in which you can request terms that you don't know how to translate, or do a search for them. But I wouldn't take all the results 100% because the person "approving" the translation is not always a professional translator and I have found sometimes very bad translations. But it does help to get some ideas.

IATE (InterActive Terminology for Europe): Is a dictionary that contains pieces of text translated by EU's official translators, so you can always be sure that you find the "official" term. There are also examples and ratings of how reliable the translation is. Well worth having it open when you are working with your translators.

EuroVoc: The EU's Multilingual Thesaurus. It is a multilingual, multidisciplinary thesaurus of the activities of the EU and the European Parliament in particular. It contains terms in 22 EU languages (Bulgarian, Czech, Danish, Dutch, English, Estonian, Finnish, French, German, Greek, Hungarian, Italian, Latvian, Lithuanian, Maletese, polish, Portuguese, Romanian, Slovak, Slovenian, Spanish and Swedish), plus Croatian and Serbian. 

STYLE GUIDES
Interinstitutional Style guide: It is the official Style Guide of the EU, and you can find it in the 21 official languages of the EU Member States.

Microsoft: Official Microsoft website with Terminology and Style guide for the languages they use to translate their products.

SPANISH DICTIONARIES
RAE.es: I think this is one of the most important dictionaries a Spanish translator must use, as all the grammatical and spelling rules can be found there. Also is useful to make sure you find the perfect word for the right definition.

Diccionario de sinónimos de la Universidad de Oviedo: Very simple, but very useful.

Diccionario de sinónimos de El País: I use this dictionary a low when I don't want to repeat the same word once an again. It also helps me sometimes when I cannot find a direct translation of a word in English but I do know the antonym word.

Spanish proverbs: Aren't sure about what that "made-up-sentence" means? Have a look here. There aren't all of them, but some of the most used.

Spanish, English, Latin, German... Proverbs: A few more Spanish proverbs, and also a few more from other languages.

Wikiquotes Spanish proverbs: You will never have enough reference.

Diccionario de rimas(in Spanish): The perfect dictionary for those who love poetry but can't find the right rhyme for their words.

Write Exppress, Online Rhyming dictionary: A complete rhyming dictionary for poetry and songwriting. It shows combinations of words, depending on the chosen criteria.

Write Rhymes: A more interactive and easy way to find the rhyming

OTHER TRANSLATION TOOLS
Diccionario de colocaciones: It explains how you can use each of the words that it contains. Yes, I know, a bit difficult to understand, but as soon as you start using it, you know what is for

Recursos varios: A complete page with resources for translators in several languages.

Localization Information: A complete list of everything a localiser needs, from what types of tools you can use, to dictionaries, glossaries, articles...

*Automated translators
Sometimes, when I am localising a game or a website, I receive other languages and, in those cases that the English doesn't seem very clear (probably because it has been badly translated from the original language), it helps if you have a direct translation from other languages. In order to do that, I use free translate online tools:

Googletranslate: The most famous free online translation tool, after Babblefish (which, to me, is not as accurate) and useful if you want to have a rough idea of what a text says.

Traduzco!: A new tool that uses google technology to translate.