Atomica Slingshot is great for this sort of thing. It took me forever to remember what it was called, as I haven't used it in a few months, and they've changed from GuruNet to Atomica, then to Slingshot (although keeping the Atomica name). Anyway, the free version lets you use the dictionary, spell check, thesaurus, acronym, and encyclopedia. Quite handy. Not to mention the full, paid version has many more features.

I now have it re-installed.