I came across this a short time ago and tested out the demo which made me like it so much I had them buy it at work for use. Built all in PHP and works on Apache and w/ MySQL (on Win or Linux, tho if you run Apache, MySQL and PHP on Windows you should be smacked as you might as well run Linux at that point).
http://www.hybodus.com/
Its not open source (unfortunately) >:(, but its a good system. Considering other alternatives dont have as many features for as cheap ;D. Ive managed to work my way into it a bit despite it being encoded code (without being hellbent on trying I might add, so yes I could do more but I see not a lot of need) and wrote other modules and scripts to work hand in hand with it.
The guys who are behind this are big on supporting it on Red Hat :P, but im writing some stuff for those of you who want to run it on SuSE, Debian and BSD, and when im confident my stuff is stable enough I'll post here for all to download.
We use MRTG for all that stuff--though I'm sure you've heard of it. It's free and works on Unix and WinNT.
http://mrtg.hdl.com/mrtg.html
Using Nagios... takes some configuring to get it right, but it's decent and open source...
Open source is always nice. I just hope this specific one isn't 'Nag Input/Output System' ..