It's hard to believe, but even in this day and age some websites insist that you must use some version of Internet Explorer before you are allowed to enter, even if the site uses no special features of Windows or IE (e.g. ActiveX).
The Switcher comes with a default set of browser ID strings: IE6, Opera, and Netscape, I believe. But what if you want more? What if you need to see if a site will recognize IE on a Mac running OS 9?
Thanks to Matt Parnell's post, I've found a site which will solve all your problems. Just go to Tech Patterns, download this file, and install it using the directions in Parnell's blog. Now you have a gazillion (more or less) browser possibilities. The Tech Patterns forum has some suggestions as to how to use them.