Within any text entry field, pressing Shift-Cmd-; (Command-:) will force that text field to be checked by OS X's built-in spell checker. I find this useful when I need to quickly figure out the spelling of a word. If I am in Safari, I just type it into the search field and hit Shift-Cmd-; and know instantly (without selecting and control-clicking) whether it is spelled correctly or not.
[robg adds: This is a documented keyboard shortcut (in the Edit menu), but the use of it within a text entry field makes it unique enough to publish as a hint. It should work by default in most any Cocoa application, and certain other applications that support OS X's text handling features.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060614142719250