Traditionally this has been called browser sniffing and has been used so that you can decide which code to use with the browser. The way you learned in the beginner's pages is much more reliable for handling JavaScript incompatibilities across browsers but you might still want to find out what browsers are being used.
alert(navigator.userAgent);
The results might surprise you sometimes which is why browser sniffing is of limited use. Also try appVersion appName, appCodeName, appVersion, oscpu, cookieEnabled, browserLanguage, onLine, userLanguage, javaEnabled and platform.
Other useful information
You can also find out other useful things using JavaScript such as the resolution the user has on their system:
alert('resolution is '+screen.width+ ' by '+screen.height);
Also try screen.colorDepth, availWidth and availHeight.
