A time will come when you simply can’t get a CSS-based design working well across the browsers. While there is still debate on the potential side effects of using them, CSS hacks and filters will frequently get you out of a jam. When used wisely and judiciously, they can be a life-saver.
To that end, Chris of Dithered has put together an astounding compendium of more or less everything that is currently known about targetting browsers and hiding CSS.