Language Extensions

BrowserDetector()
You can use the BrowserDetector function to isolate the potentially useful portions of the User-Agent string.
All JavaScript browsers

MD_random()
This function returns a random integer between two numbers.
All JavaScript browsers

Array Enhancements
JavaScript 1.2, which works only in Netscape 4.0, adds several fundamental methods to the array object. A few of these very useful methods are included here in a form that can be used by 3.0 browsers.
All JavaScript browsers

Plug-in Bot
There's no fool-proof way to detect plug-ins. But this is a pretty good start.
All JavaScript browsers