My main go-to is, quite naturally, Rikaikun (FF)/Rikaichan (Chrome). Hover dictionary. Indispensable for a quick look-up.
The newest addition to my arsenal is Readlang, which also comes as an extension. While not optimised for Japanese, it works by on-the-fly translating (I think using Google, but I might be wrong -- at the very least, the dictionary it draws has more frequency-slanted usage than Rikai’s static one does) words that you select. Grab an account and you can save them and get them automatically turned into flashcards. It’s a lot of what I used to do manually turned into a sort of app, and while there’s plenty that can be improved (the interface is a little clunky, I’m not a huge fan of the colours/fonts, Japanese support is nil which means you’re on your own for kanji readings), it’s extremely useful for forcing yourself to read, review, and re-read. I splurged and got the unlimited version for $30/year, which is way less than the amount I’d probably spend on notecards, pens, printer ink, and tears.