Skipping markup is pretty much the main advantage (including url checking and \tt skipping). Excalibur also allows adding a lot of dictionaries so it makes it easy to check in several languages.
http://www.eg.bucknell.edu/~excalibr/excalibur.html
I look forward to your implementation though. Sounds great!
********************************************************************* Allan Odgaard wrote on [Monday, February 28, 2005, +0100]
I don't know excalibur, but what advantages does it have over the system spell checker that TextMate currently uses? If the only advantage is that it skips the markup, then this should be solved in beta 6/7 where it'll (amongst others) be possible to selectively disable spell checking for elements matched by the syntax definition.