Lingro: Look up definitions, translations on any page
The definitions are culled from publicly available dictionaries and user contributions filed under a Creative Commons license. So if you find a word without a definition, a message will pop up asking if you'd like to contribute one. Of course, the odds of your clicking the word if you already knew the meaning are pretty slim (unless you're say, writing a review of Lingro).
You can also use Lingro the old fashioned way, by visiting the service's home page and typing in a word or entering a web address to translate. There's even a service that lets you upload a file from your desktop for translation into another language.
Honestly, we didn't have much luck translating entire web sites. But Lingro's dictionary definitions and single word translations seem pretty good.