Open Source (Linux-) Server monitoring Software with lightweight footprint

What OSS monitoring soltions are there? I know only nagios & cacti.

Are there any real live performance monitoring tools?

From serverfault Martin K.

• You can take a look on Monit.

  http://mmonit.com/monit/

  From Ellendhel

• Have a look at http://www.zabbix.com

  It's not as fully featured as Nagios but its improving pretty rapidly. Its quite lite weight both in terms of server load for output and the collectors too.

  DisabledLeopard : If you do have the spare cpu & memory consider ZenOss Core http://zenoss.org - after you get used to its user interface its really nice to use and has a lot of very cool features [like different levels of user accounts, reports etc] that so far I've only seen available in commercial apps.

  Martijn Heemels : Zenoss Core is great (my company uses it), but it's not lightweight.
  From DisabledLeopard

• for trend tracking - munin. you can use it as well as source of alerts for nagios.

  From pQd

• The monitoring agent, sd-agent, for our hosted monitoring service, Server Density, is released under the open source FreeBSD license and is designed to be very lightweight.

  Martin K. : It's leightweight and neraly exactly what I've searched for. But only for one server. If you want to monitor more you have to pay! That's no open source solution.

  DavidM : The agent you install on your server is open source (FreeBSD license). The monitoring is provided through our hosted service, which is free for 1 server (& limited functionality) and then chargeable for more features and servers.
  From DavidM