the best translation add-on, period Rated 5 out of 5 stars
By far the best translation add-on among quite a quite a few that I've tried. When translating the entire page, it does so in place, like Google Chrome does, rather than redirecting you to the google translate site where your page is framed and may lose functionality. It even automatically translates any new text that appears dynamically on the page (ajax etc.). And you can hover to get original text where you can see alternative translations of the pieces, just like in Google Chrome or in the translate-some-text feature on google translate site.
If you want a simpler add-on for similar page translation only, try gTranslator by GHR, which, however, requires that you explicity click to translate each page. There's a bookmarklet that does similar to gTranslator (from http://googlesystem.blogspot.com/2009/11/google-translation-bar.html ).
All appear to be based on the old google toolbar translate feature and invoke similar code from the google translate api site. All of these use the unsupported google translate public api, so they may stop working at some point, but in the meantime they're great.
I would love an option to invoke this automatically on *every* site except those specifically excluded, but if they are detected as already being in the target language then do nothing.
Unfortunately, the page translate feature of this add-on is blocked by Flashblock.