[samuraicatpizza]

If you use Firefox, the gTranslate extension is pretty good for looking up words/phrases quickly. All you have to do is highlight the word, right click, and there's a translation menu item that has the translation. It might not be the best for individual characters, but it's quicker than going to the dictionary every five seconds.

[bemmu]

http://www.rikai.com automatically displays popups for kanji as you hover your mouse over them. Here's a direct URL to browse the Japanese Slashdot with it: http://tr.im/ooJg

But what I really meant were dead-tree books and newspapers that I could read while not at the computer.

[xiaoma]

The peraperakun plug-in gives pop-ups for words and phrases and it was written specifically for students of Japanese and has been around for a long time.

https://addons.mozilla.org/en-US/firefox/addon/3343