Google has one: https://translate.google.com/. Microsoft has one: http://www.bing.com/translator/. And there are language learning sides like http://www.spanishdict.com/ that also offer a textbox to translate stuff. Additionally you might want to check whether you cannot simply add some dictionary as a search engine to your browser, Firefox for instance allows that for dictionaries like http://www.wordreference.com/es/.

You can just key in the word in an English Spanish dictionary. :)