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?

>>9 I like this idea. Think I'll go with it.
>>10 How would I integrate the where command into my java program? Seems like that might get a bit clunky.