Browser is a global object that contains markers allowing you to determine the current type of browser. The following keys are available:

IEInternet Explorer of any version
OperaOpera browser
WebKitWebKit based browser
GeckoGecko based browser
MobileSafariMobileSafari browser
KonquerorKDE default browser
OLDan internal marker for IE 6 and 7

Usage is pretty much straight forward:

if (Browser.OLD) {
alert("Welcome to the hell");

Check the browsers page for more information on supported browsers.