The most popular web hosting software is Apache, and with good reason too. It is free and a great software package. While most people are pleased with its mod_php capabilities, Apache 2.2 has so much more to offer, and a lot more in the pipeline!
Previously Apache 2 users would have to use modexpires and modheaders to configure caching, and there wasn't much middle ground between the back-end and the client. Now with modcache, modmemcache, and moddiskcache, the possibilities are awesome. With modcache, the HTTP caching specification really comes alive, and not only can you improve upon client revalidation checks, but you can also cache compressed data on the server for serving to different clients. It even saves different versions for different user agents as well as different cookie sessions (good for logged in users).
Along with modcache, setting up redundant web servers is easy - just proxy requests via modproxy or modrewrite and cache the results locally. The power of modrewrite, modproxy, and modcache might not seem like a lot, but it is huge.
The benefits of modrewrite have been around for awhile in terms of Apache, and one of my favorite capabilities is the prg: function, which allows you to use a program to access a stdin parameter and return a rewrite target. Personally, I use it for keeping track of all my URLs, as well as redirecting domains, but it can also power some pretty amazing virtual host environments. For example, you could setup your virtual host environment in a relational database. This is something that lighttpd has had for awhile in modmysql_vhost, which Apache didn't have.
While not ready for prime time, moddbd is a module with a lot of potential. If you've ever been frustrated by the convoluted process of authorizing via modauthmysql or modauthpam, then you should be pleased to know that moddbd will save the day. Along with aprdbd, moddbd can use various database specific drivers (postgres, mysql, etc.) and maintain persistent connection pools to the server, increasing efficiency bigtime.
While the initial use of moddbd will be authentication (authn) and authorization (authz), I'm hoping to see moddbd power a more tightly coupled relational database powered virtual hosting system than the one I mentioned earlier.
Previously I mentioned modphp and how a lot of people (including myself) have been using that module as a mainstay in their Apache experience. With good reason, Apache and modphp make a great pair. I have started to migrate away from mod_php though, thanks to fastcgi. I first learned about fastCGI through lighttpd and trac, and have really grown to like it a lot. One really nice thing about it is that it allows you to run Apache using the mpm worker module rather than the mpm prefork module. That's a very good thing for efficiency and performance as well.
Those few points I mentioned above are really only the tip of the iceberg when it comes to the greatness of Apache. If you have any favorites that I missed, please comment! Thanks for stopping by.