I tried looking to see if this had been plugged yet in this guild, and I couldn't find any mention of it, so I figured I'd do it myself.
There's an addon for Firefox called "Rikaichan" that has been an
immense help with me in learning and understanding the language. It's basically an in-line Japanese-English dictionary; you mouse over a word, and a tooltip pops up with all the possible translations for it. It's accurate enough that the first choice is usually the correct one, at least in my experience.
This is incredibly useful if you've got a good handle on grammar, but your vocabulary and/or kanji skills are lacking. Both of which are true for me.
It only works on raw text, though; so it's nigh-useless on sites like niconico that use flash elements and the like.
You can get it
here.