What programming language are the largest websites written in? Such
Amazon, Google, eBay, MSN, etc.

Many of these sites use ASP.NET
Some use PHP and even fewer use ColdFusion by Macromedia.

You can use the following to see what a site is running:
http://uptime.netcraft.com/up/graph

Amazon -> Linux
Ebay -> Windows Server 2003
MSN -> Windows 2000

Usally if its a windows machine its either ASP or .NET.
Linux, FreeBSD or some other unix its PHP or it can be JSP.

But use the uptime.netcraft.com and you can check it out for yourself.

What platform the websites are run on doesnt necessarly dictate what
programing languages are used.

The easies way to tell (for most websites) is by going to a page and
looking at the extension the page is using. This is called a MIME
Type.

if .php is in the URL: Its Written in PHP
if its .asp: Then its active Server pages
jsp = java server pages

etc.

In the case of MSN. I pulled a random link from the msn.com main page.
http://autos.msn.com/default.aspx?src=autolink

The Site is written in ASP. Active Server Pages.
Youll find that most of the largest sites are in ASP.

Amazon and eBay are written in PERL and run on Linux.

MSN is by Microsoft policy done in Microsoft technologies.
Google has many projects and they are written in many different languages.

Generally PERL is very popular for large scale HA web commercial
deployments. And despite all the new trends as php, python and others,
it is still the most commercially used language on web.