Website uptime checker

[kapow]

We manage about 70 websites. Sometimes customers tell me their site is down, then I find it was a problem with their ISP not the site/host.

Can anyone tell me a system that:
- Periodically checks multiple sites to make sure they are up and running ok.
- Email me if a site is down.
- Keep stats so I can show a customer their site was up when they thought it was down.

[bakedjake]

Free: nagios - [nagios.org...]
Paid: Keynote RedAlert - [redalert.com...]

I use both, and love them both.