One that I've really grown to appreciate is TinySpell
. It sits on the Quicklaunch bar and runs in addition to any other checker that might be loaded. Since it works directly with the keyboard buffer, it doesn't care what program might be taking input. It just monitors every keystroke and alerts you to a spelling that doesn't match one in it's 100,000+ word list. Mispellings (like that one) make its icon go yellow. If you click on the icon, it gives you a list of spelling probables you might have meant. Click on the correct one and it automatically replaces the mistake.
Simple, free, and small, no dictionary or other devices.