Web Servers 

Apache Web Server

Also called httpd. This is the de-facto web server these days. Note that "apache" refers to the apache foundation now. The apache foundation manages a large number of other open-source projects like "apache tomcat" (a servlet engine).

LightHTTPD

(nyt)

Content Management

Gallery2

Excellent software for storing and displaying photos on the Web. I use it for my photo site: http://www.cholul.com. Works very well even for large numbers of photos (I have over 17,000).

Drupal

Content Management System (CMS)
software. Through the use of modules it provides a great deal of useful
functionality such as blogging, forums, search, books, etc. I use it
for this web site.

MediaWiki

A free wiki originally for Wikipedia. Installs easily. Works well.

WordPress

Very good blogging software. I used to use it until I had a need for additional (non-blog) content management.

Alfresco

Looks very promising, but is very resource intensive and the current community version has some issues.

Related

Utilities

• VandalSniper - Scan for vandalism in content.

Links

• List from Wikipedia

Items marked with (nyt) indicate I have not yet tested it. I've heard
it's good (so it has made it on the list) but have not yet tested it
myself.