Good digital dictionary?

I need a good dictionary for checking whether or not a given string is an English word or not. Do I need to approach this problem a certain way? I'll be needing to scan a lot of words, so I'm prioritizing quick lookup time.

If it matters, I'm working in Java. Suggestions?

>>7
Think of a nice utility: the Windows equivalent is there isn't any.

>>6
If you're doing this multiple times within the running time of program then it might help to keep them sorted in memory as an array and then just binary search on it. I mean, if you're using Java you can spare the mem overhead