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?

>>4 Very funny. Seriously.
>>5 I suppose that will work? It's just newline delimited, which seems like what I want. Will searching that be efficient if I'm checking every word in a text file?