An IT Stack (for the purpose of this discussion) is a server operating system, web server, database server, and a development language. EWeek Labs has done an interesting comparison of the .NET IT Stack to some Open Source IT stacks.
On the .NET side - Windows Server 2003, IIS, SQL Server 2005, ASP.Net and SharePoint Portal Server 2003.
On the Open Source side - some Windows servers, some Linux servers, some Apache servers and some Zope application servers. Some databases were MySQL, some were ZODB (Zope Object Databases) and one Hypersonic SQL database server. The Open Source platforms ran on JSP, Python, and PHP. The portals chosen were JBoss, Plone, XOOPS, and Liferay.
Let me say that I'd pretty much never heard of most of what was going on on the Open Source side, but that's what this article (and this magazine) is good for - showing comparative analysis between familiar products vs unfamiliar products.
Check out the article to see specific details, but the long and short of it is that the .NET stack was perhaps the most performant combination of products being reviewed. A mix of Windows OS + some open source software did very well too.