Pretty much all web developers should know by now that browser sniffing is evil. If you don't know why, you should definitely read Richard Cornford's excellent treatise Browser Detection (and What to Do Instead). Feature detection, where you look for the specific feature you want to use, is much safer; taken to the extreme, it can end up like the rather clever Modernizr project. But what if you really do just want to know if your code has the misfortune to be running on IE7?