Collectd is great. High resolution data collection, reliable, low resource usage. We wrote our own rrd graphs against it.
What do you use to generate graphs?
collectd can spit out rrd files, and there are tons of rrd graphers out there.
I use drraw... it's really old and boring, but I like it for it's simplistic and I am used to it. (it's just a perl script you run on your web server, no DB needed)