Probably one of the best websites from the EU for translating complex terms, acronyms, etc.. covering most specialised areas.
Multilingual terminology database from the UN
Specialist dictionaries for translators
Business Glossary - English-Spanish, Spanish-English business glossary, with over 8,000 items.
IT terms - defines thousands of IT terms.
Film Glossary - Excellant movie database.
Law Dictionary - To translators, the dictionary is probably the most useful of Law.com's extensive resources.
Legal Dictionary - Nolo.com has both a legal dictionary and also an encyclopaedia on legal topics.
Quotations - Searchable database of quotations in English (also dictionary and encylopaedia).
Slang - Dictionary of UK slang. Try their links section for other forms of slang (Australian, US, specialist...).
Webopedia.com - Online dictionary and search engine for computer and Internet technology. Results generally excellent, as are the links provided.
Chartered Institute of Linguists - (CIoL) runs translator and interpreter exams for those wanting to qualify as a Professional translator. Their handbook and exam registration is available online. DipTrans preparation courses can be taken at University of Westminster, City University and International House Barcelona (online course).
Institute of Translation and Interpreting - The CIoL's main rival has some interesting advice on getting started as a translator on their site.
American Translators Association - The ATA is the leading US translators association.
TRIAC - Professional Catalan Association whose objective is the creation of a Col·legi Oficial de Traductors i Intèrprets de Catalunya in order to regulate, protect and improve the profession.
Language directory - Translation and language related companies and individuals.
Written Language Guide - Good Written language guide of European languages and their differences. Alphabets, samples of translations and general cultural customs.
Useful Links for Translators
Excellent discussion area for translators on everything from whether you should provide free sample translations to being paid less having forked out on expensive CAT tools.