Depending on the request made, server responds back with specific details which can range from browser specific css layout to calling certain javascriptsfor certain browsers.

Changing the user-agent also helps us in understanding how spider-agents or bots will see the pages that we build. It also helps us design applications that will mimic a request as originating from a mobile phone and helps us in viewing the behavior of the application in different mobile clients.

To read more about HTTP-Headers check ietf

Various scenarios can cause memory leaks which can range from unhandled memory garbage to cyclic references to error in code logic. One best suggestion given in the javascript forums to overcome this problem is to nullify the element once its usage is over.

A simple and readily available tool to check on the memory leaks in the system is the Windows Task Manager » Performance to visually monitor the effect of cyclic references or unhandled situations in your Javascript code.

Another tool is Leak Monitor extension for Firefox.

Screencast on IE Memory Leaks can be viewed at Barelyfitz.com

If you are interested in learning Javascript you can watch a series of lectures from Yahoo Video Library on Javascript by Douglas Crockford. Some interesting points like (i) is NaN a NaN? ii) What is the type of Nan? are discussed in his lectures.

Another interesting article on Memory leakage pattern.