I believe there's an extension for the Mozilla browsers that allows this. However, changing user agents isn't recommended. Especially if you're changing it to that of a different browser. A lot of web sites use scripts to generate browser-specific content.
Opera, K-Meleon and KDE Konqueror have this feature as standard, there is an extension for Mozilla and Firefox which does it, Safari (in Mac OSX) has an optional debug menu where you can do it, and even the text browser Lynx and the wget download tool have this option. In IE you can change the string at least partially by editing the registry. So, the shorter answer is: almost all modern browsers can spoof the user agent string.