Rikaichan is a Firefox plugin that acts as a Japanese reading aid; I hover to the left of a word or phrase I don’t know, and possible definitions pop up. While I may be using it a bit too much as a crutch, I’ve found it really helpful with quickly confirming that I’m reading something right or in deciphering kanji I don’t know without copying and pasting into a dictionary.
One of the big reasons I love Rikaichan, though, is that it is totally up with slang. For an excellent example, click the image below!
Oh, Rikaichan. ワロタ indeed!