But when it comes to language learning its pretty obvious, but this is a real world tool which is functional outside of academia. Living in a multi-language world mandates this.
I have tree input methods on my computer, covering latin, cyrillic and Japanese scripts - and use all three regularly. On the road, or plane, an online source, or the heavy book shelf is not always available, so there needs to be a good local source.
For words and phrases I've fallen for the new Dictionary in OS-X Leopard. Used a variety of tools in the past, but this has been quick and fairly good of late. The only problem is that by default you don't get very much and its very easy to sweep it off to the side for ever.
The solution is its dictionary expansion. David Tse did a great writeup on expanding Dict App as a resource.
The other local tool I get to often is JEDict. Also expandable, but created with Japanese in specific in mind. Runs on practically every Mac OS version - from MacOS 8.1 (Release in 1997!) through 10.6 !
Have fun with quick reference!