You can get whole Web pages translated (Google Language Tools, Babel Fish, etc.), but here’s a free tool to translate any individual word on a Web page you want: Lingro (via Education Week).
Lingro works for English-English and both to and from English for:
- Spanish
- French
- Italian
- German and
- Polish.
It uses the free Wiktionary dictionaries and also has an input-translator dictionary, file translator, and vocabulary study tools.
Go forth and click multiple unknown words.