Linux based monitoring solutions
Let's assume you are given a Windows network to monitor. Exchange, SQL, File, Web, etc.
You want to monitor event logs, system status, available disk space, security, etc, etc. Preferably with filters, e-mail/pager alerts, reports etc etc.since you won't have time to go read logs.
You want to spend $0 but you don't want to have to spend all your free time learning how to reinvent the wheel.
Which products would you use, running on which distro?
Be nice, be logical, be ready to define your terms...