InfoWorld's 2009 Bossie Awards
Nagios is just about everything a system or network administrator could ask for in a monitoring package. The Web GUI is fast and intuitive, the back end is extremely robust, e-mail and SMS notifications can be managed very granularly, and plug-ins exist for a massive array of hardware and software -- almost anything that speaks IP. Nagios' immense capabilities can present a significant learning curve. Read the review.