Free software or something you're prepared to spend some money on?
Free:
Cacti or Munnin probably the easiest to set up, cacti being mostly snmp based and Munin having the option of using an agent.
Things like the community editions of Zenoss / Nagios also probably worth a look as they would let you collect a lot more information but can be a bit of a pig to set up imo.
Paid for stuff:
We use Nimbus from Nimsoft, its not cheap but it allows us a lot of flexibility that I've not seen in other products.
There's also the traditional offerings from Solarwinds etc.