Here's a nifty tip that should work in all Cocoa apps (Mail, TextEdit, Safari etc.) If you can't remember how to spell a word, type the beginning and hit Escape for some word suggestions.

There are a number of different ways of doing this however. In certain places it isn't possible to press escape, because it does something else (like clearing the Google search box in Safari). In these cases you can use Option-Escape, or just press F5. These two alternatives may take a little longer, but seem to work everywhere.

So now if you can't spell something, you don't have to attempt the whole word, just put in the first few letters and let the system spelling dictionary do the rest.