A lot of folks ask us how to "download" IIS; since other web server offerings are available for download on a multiple of platforms, it's an obvious question for the Microsoft web platform stack as well. The answer "for now" is that you can't download IIS because it only comes with Windows.
Well, IIS has historically shipped as part of each operating system, and we have never back-ported IIS from one OS to another. The exception being that we have ported certain features back in our Out of Band modules (extensions) as we like to call them.
Now that we have shipped Web Application Gallery, you can install IIS easily through that tool.
So let's break it down by IIS version and operating system. Plus I have some comments on my experience with these versions:
IIS 4.0 - NT 4.0, Option Pack 4
I remember when I was a teaching assistant at WWU in Bellingham, WA, we were using IIS 3.0; and then I read that IIS 4.0 came out; after talking my professor into it, I installed it and wow, the stability went way up! Now I didn't need to reboot the server before class in hopes it would last 60 minutes. :)
IIS 5.0 - Windows 2000 (Professional & Server SKU's) (use Add/Remove programs -> Windows Components)
When I joined Microsoft, I was part of a team busy doing a re-architecture of IIS (the genesis of the IIS 6 project) while another team finished up IIS 5 and shipped it. This release was a tough one, because although it added some great features in terms of manageability, it also suffered from a very nasty series of security vulnerabilities. The IIS team took this very seriously and has to date never forgot the lessons learned. The early results from that day were URLScan and a super-focus on security. Today, we still do security reviews of every little feature in IIS and we're constantly trying to break our server. Also, we stopped installing IIS and it's sub-components by default after this release.
IIS 5.1 - Windows XP (same as IIS 5 - use Add/Remove programs -> Windows Components)
I remember being frustrated that we had to ship IIS 5.1 (basically a big bug fix release of IIS 5.0) on XP ... I wanted to let everyone see IIS 6.0 because it was so very stable and extremely secure; the timing just didn't work out in the ship schedules. The other goofy thing is that we actually shipped IIS6 on the 64bit edition of Windows XP which apparently hardly anyone uses.
IIS 6.0 - Windows Server 2003
IIS6 was a great release, we spent about 2 and a half years building it; during that time we created a great test infrastructure that we still use to find regressions today. We became the poster child for how to do secure coding by defense in depth. In 6 years, we only found 6 vulnerabilities but nothing that was "worm-able" like in the past. In fact, hackers turned to SQL injection attacks and application-level attacks on the Microsoft platform, attacking poorly written ASP pages and SQL scripts.
IIS 7.0 - Windows Vista SP1 & Windows Server 2008 (UI) or Windows Server 2008 (command line)
IIS7 was a big change in terms of setup complexity, configuration, and UI. All three of these changed drastically to allow for complete customizability and extensibility of the web platform. Now you can add your own components to IIS and get full-featured management support in terms of integration with our configuration and most of our administration tool such as appcmd and our new UI.
IIS 7.5 - Windows 7 - coming soon!!!
IIS7.5 is a minor release for our team, basically we integrated several recent out of band projects, added several cool features and of course fixed as many bugs as we could. Most of the team is working on our IIS extensions and the Web Application Gallery efforts as well as the PHP community efforts. Still a very cool team to be on and one I hope to stick with in the years to come!
IIS 8.0 - ??? - it willl blow your mind! :)