What are your favorite online Japanese study tools?

I use rikaikun (on Chrome) extensively (aka rikaichan on Firefox). I especially like that you can use keyboard shortcuts to hide the translations, so you can just use it to get kanji readings but still challenge yourself to learn the vocab.

I’ve tried other programs, like plugins that insert furigana into pages, but I always end up coming back to rikaichan.

I’ve used japaneseclass.jp to brush up on vocabulary as well. It’s a pretty decent tool and easy to use.