I'm sure lots of you know about these links, but I also think those new to the profession might not, so here it goes. I use these websites very frequently when translating.
Acronym Finder: http://www.acronymfinder.com/ - Contains over 4.219.000 acronyms and abbreviations
OneLook Dictionary Search: http://www.onelook.com/ - One Look is like the "Google" of the dictionaries. It will find definitions in several dictionaries such as American Heritage, Merriam-Webster's,
Cambridge, Oxford and Wikitionary
Convert me: http://www.convert-me.com/en/ - Online conversion for many measurement systems
Merriam-Webster Visual Dictionary http://visual.merriam-webster.com/index.php - A great visual dictionary - more about it here: http://translationblogsdigest.blogspot.com/2009/06/visual-dictionary.html