[lil_cain]

More than you can shake a stick at.

For graphing: Munin, Cacti, Graphite, and ganglia are the normal options. Graphite is the most powerful of these. It doesn't do any actual monitoring though. Munin is the simplest to implement.

For general monitoring and alerting: Nagios (and its clones: Shinken, Icinga, and Naemon) is the standard. There's also newer projects like reconnoiter (which suffers from awful documentation, but looks like it could be really nice), and Sensu (who's got quite a bit more documentation than reconnoiter, and looks like it could be quite good). A reasonable number of people use Zabbix as well - the main benefit of this seems to be a nice GUI for management (but I've never used it, so take my comment with a pinch of salt). Graphs (and almost anything you want) can generally be added to nagios and clones if you're willing to do a little work. Reconnoiter seems to have graphs out of the box. I don't know about zabbix or sensu.

There's more tools that you can use for log monitoring - I think that's a whole separate area, but if you're interested, ping me and I can point you in the direction of the right tools :-)