Heres an extension for Mozilla Firefox that allows you to change your user agent at will to anything you choose. I have include a list of dozens of known UserAgent strings.

Heres a link to several others including the extension posted here, lots of great stuff including an extension that helps to ensure that you havent visited a cloned site while somone went phishing for your info.

Mozilla Extensions

hmm considering all the shit looking people do @ the office perhaps installing this will help from info leaking to the wrong hands. Thanks alot.

proxomitron will do this & more

and here's some info on changing it in IE as well ;D

http://cal.phonelosers.org/forum/news170.htm

good info, works good

I Prefer A browser Called "MiniBrowser",
it is made exactly for such things (Faking UserAgents, Cookie Editor, Easily changing POST and GET data and more...)

Here's A Link:
http://www.aignes.com/download/minibr.zip

Enjoy...