The easiest translation browser plugin

Ubiquity is a Firefox plugin that, amongst other things, allows for immediate translation of any text you have selected in your current browser window. The plugin is under development but performs quite well for translating Russian to English (likely the other way, too). Select the word/sentence/paragraph you want translated, hit Alt+Space and enter "trans" - simple. Check out the demo video and project pages.

http://labs.mozilla.com/2008/08/introducing-ubiquity/
http://vimeo.com/1561578?pg=embed&sec=1561578

Download link:
https://ubiquity.mozilla.com/xpi/ubiquity-latest.xpi

Looks very promising for my translation needs - sure beats hopping to Babelfish or Google Translate just for a word or two.

EDIT: changed download link, was having problems with the old one.

It only works for FF 3.0 and up, unfortunately for me.