Hi!
You probably know that a while ago Microsoft made a special page for all those clients who had Netscape or Opera. It simply displayed "Please update your browser to IE so we can make friends again".
Seriously, this went on for a few days before Microsoft had to change the policy and admit those users. With an attitude, though...
Point is, it is possible to redirect clients to different pages depending on the client's browser. You can do it with any server side scripting language, even with client-side languages like Javascript. However, Microsoft has one site for all users, much to the dismay of people using Netscape and Opera. I tried their site with Opera, the menu in MSDN for instance does not behave the way it is intended. It doesn't even do the job on early releases of IE 4.0.
The menu is not the only thing they've introduced to keep off those pesky users who did not use their browser. For instance, the Search page in the Software section uses 100% DHTML to retrieve search results without reloading the page! Of course it only works on IE.
IE is a great browser that uses a lot of unapproved extensions to HTML and Javascript specs. There are incompatibilities in DHTML, CSS, Javascript; it also uses proprietary VBScript and ActiveX technology in the browser. This does give you a lot more freedom in coding - you can even do a nice HTML editor using ActiveX components like DHTMLEd from Microsoft.
DHTML in IE is actually a combination of CSS and Javascript. In contrast, in Netscape DHTML is achieved by using HTML + CSS + Netscape HTML extensions - Layers.
Jason is right, they use CSS + Javascript (or DHTML) to make the menu look so cool. Doesn't look as cool on Opera or Netscape, however.
Eventually, other browsers will die out. Opera costs money and a lot of Internet users just don't want to bother paying it (they did make a Shareware version with banners, which is a smart move).
Netscape decided their code wasn't perfect so they rewrote the engine and it took them 2 years to make a browser that wasn't a lot different for the end user (it did introduce themes and other gimmicks, but who cares for such things?).
IE, on the other hand, is getting more and more stable. If the .NET strategy of Microsoft is right and they will be able to reimplement it on Linux with the help of Linux developers, they will port the version to that system so Netscape may go out of business. Of course, the questions is - will Microsoft want to afford it? Usually this is very difficult - they have to rewrite ActiveX and a lot of other things to be compatible with current Linux Window managers (and with Linux developers jumping from one implementation to the other, will there ever be one unified Window manager for Linux - like Gnome?). Microsoft didn't even do it for Mac - and they already have IE ported to that platform.
For now, I suggest to use something achieved with simple Javascript and HTML - you can do wonders even with these pretty basic things.
Hope that helped! Sorry for being too verbose.
Best,
Stas