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?

>>11
Don't do that, I was just answering to >>9 and hadn't read the thread.
Find an appropriate data structure for quick lookup in your dictionary. A Trie, for instance.