As recommended above, Chrome is very good and offers a pop-up translate toolbar everytime a non-English web page is displayed.
I also use Firefox and have the following add-on toolbar installed:
http://www.unlimited-translate.org/
There are one-click icons for translate whole page, highlighted text or individual words.
EDIT: if you choose to use this, just be careful not to include the options for Babylon. and set Google Translate as default. Babylon is another online translator service, but it can change some of your browser default settings. However, despite this warning, I think it was only a problem for me when I directly downloaded it (Babylon - to try out) - can't exactly remember - long time back. The toolbar itself (above link) though is very good.