Apache remains the world's most popular web server, thanks in part to the abundance of powerful modules that work in synergy to extend the web server's core functionality. Here are some useful Apache modules that can improve your web server's performance, content delivery, and overall security.

Every organization must monitor its infrastructure's uptime and performance. While the popular Nagios application is a good general-purpose monitoring program that you can extend with plugins to handle just about any task, you may do even better by employing Cacti as a graphical front end to RRDTool's data logging and graphing functionality. Cacti was developed specifically to monitor and collect performance information, while Nagios is more oriented toward state changes, such as noting whether a daemon is up or down.

Nagios is a great application for monitoring your systems, allowing you to set alert levels and trip actions when those levels are reached. The software uses a plugin-based structure; even the most simple functions (such as check_ssh and check_disk) are plugins. This makes Nagios incredibly flexible; if there's something you want to monitor, and you can think of a way to write it, you can write a plugin, hook it into Nagios, and start running it. But even better than that: for most things you might want to monitor, someone has already done written the plugin for you.