Wikipedia, Dictionary.com, Thesaurus.com, tyda.se (Sw/En word translation). Those would be my most common ones. Sometimes use rhymezone.com and onelook.com when I'm in a wordplay mood.

Also, for those who don't already know it, I'll grab this opportunity to mention WolframAlpha, which is a pretty cool project in the works. I'd explain it, but it's better to watch the screencast. Or try some searches, like:
GDP per capita USA vs. Sweden
or
hematite, magnetite, chromite
or
n-grams "it was the best of times it was the worst of times"