Excalibur is a Macintosh spelling checker for plain text documents. It will spell check documents created by any text editor such as BBEdit, Alpha, Emacs or MPW. It will also spell check the clipboard, making it a useful spelling checker for mail programs such as Eudora and news readers such as NewsWatcher.
Excalibur is also a very good LaTeX spelling checker. LaTeX is a collection of typesetting macros. If you're not a LaTeX user, you should turn off the option that makes Excalibur aware of LaTeX. See LaTeX Options for more information.
Excalibur is a stand-alone spelling checker that you can use with many other applications. If you want to use it for spell checking the clipboard, it's probably best to turn on the Open the Clipboard option or the Open the Clipboard and Go option. See Start-up Actions for more information.
Excalibur supports Word Services which streamlines its operation with MT-NewsWatcher, Eudora Pro, Communicate, Nisus Writer, and ClarisWorks.