Monit: lightweight monitoring solution

Monit is simple, lightweight, but useful and powerful enough monitoring solution for your servers.
Monit can monitor:
  • OS processes (presence, resources)
  • files, dirs and file system for changes (mtime, size and checksum changes)
  • network hosts (ping, tcp connections)
Monit can notify administrator via configurable e-mail messages. It also can automatically restart failed service.
Monit have embedded web-server which allows to view state on monitoring objects and disable or enable them.

Of course, enterprise-class monitoring systems have much more features, but sometimes they are too complex and unstable.
BTW, there is product named M/Monit. It can control multiple monit instances. Unfortunately, M/Monit is only available under commercial license.