Opencores stat server?

cacti

If you need graphics, you can approach in addition to cacti and MRTG. The license is GPL.

munin
nagios is also able, but he's more of a warning system about problems.

munin, the easiest to set up.

I for munin
\rwww.odmin4eg.ru/2010/ustanovka-munin-v-ubuntu/

download channel

ntop (with schedules and full of details for the long-term statistics), iftop (severe console for realtime tracking)
\r

some processes

top, htop, iotop to real-time statistics you have suggested. I will only add that if a lot of servers, we can already think of so big a fellow as zenoss/zabbix/etc. I use Zabbix, it's gorgeous.

habrahabr.ru/qa/1045/ — in the comments view.