FreeSpell is a spell checker for any and all Windows applications. You can use it in Eudora, in Microsoft Outlook, Firefox, Visual C++, and even in Windows Explorer when renaming files (though it's a little tricky to do that).

In fact, anything that supports the clipboard supports FreeSpell, and all it takes is a hotkey to invoke it. Just as important, FreeSpell is based upon the Aspell engine, which means it offers the best spell checker around for people with weak spelling skills (as well for people who just need a quick proofread). Aspell is full featured as well, with a large default dictionary, and custom dictionary support.