For Mac OS X users out there, Leopard contains Japanese, Japanese-English, and Japanese Synonyms dictionary built in now. It's actually very nice. I use Pages as my Word Processor, and it provides a right click option that will look up a word in the dictionary. Mind you, I'm still a beginner, but when I'm going through my "daily" lessons (aside from my regular JapanesePod101 fix), I find it quite useful.
The Japanese-English lookup is quite good in that it gives lots of example sentences to show how to use the word.
For example, I wasn't familiar with 気持ちat first -- until I looked it up and remembered hearing it in one of the lessons. The example sentences include (in English):
respect others' feelings
he felt friendly towards me
how does it feel to be a mother
i can understand why he got angry
i wish you would try to understand how i feel
i don't feel like attending a dull lecture
Anyway, if you are a Mac user, and you haven't upgraded yet... this is just one more reason to consider it.
