RPi-Monitor is a self monitoring application designed to run on Raspberry Pi. It provides an interactive web interface to display status and graphs. Screenshots are available here.
For performance and security reason, RPi-Monitor separates the extraction of the information from the presentation of the information.
The extraction of the information is done by a process designed to run as a daemon (which can be executed as root). The extracted key performance indicators (KPI) from the computer are stored them into a Round Robin Database (RRD) to keep an history of the health of the computer. rpimonitord is the perl script also starts the embedded web server giving access to the pages. The web server is running into a separate process owned by a non root user (the user 'pi' by default).
Since version 2.0, RPi-Monitor extracted data and informtion display are fully configurable. Some articles (available here) are describing how to extract data and how to customize their display into web interface.
Finnally note that the embedded server doesn't provide access control or authentication. It is still possible to not start the embeded web server and use an external web server to deliver the pages. An article available in the blog of the author is explaining how to setup ngnix to manage access control or authentication.
You will find the latest information about RPi-Monitor on http://rpi-experiences.blogspot.fr/
Since version 2.6 RPi-Monitor is available in the Pi Store 
Before installing RPi-Monitor you should install the dependencies. To do so, execute the following command:
sudo aptitude install librrds-perl libhttp-daemon-perl libjson-perl libipc-sharelite-perl libfile-which-perl
Download RPI-Monitor debian package from https://github.com/XavierBerger/RPi-Monitor-deb/tree/master/packages and install it with the following command:
sudo dpkg -i rpimonitor_X.X-1_all.deb
That's it, your Raspberry Pi is monitored. You can now browse http://your_Raspberry_Pi_address:8888 to access to the interactive web interface.
To remove the package you can use one ot the following command. If you want to remove the program but keep the configuration files and statistics:
sudo dpkg --remove rpimonitor
If you want to purge everything related to the program:
sudo dpkg --purge rpimonitor