Free Site Uptime Monitoring: Mon.itor.us
Downtimes seem to be the nature of the hosting & internet business. But there is a limit for it which you must measure whether your website’s uptime is below or above that.
Mon.itor.us is a free site uptime monitoring service which warns you when your website is down and provides statistics of your uptime & downtime.
It monitors your website from different locations 24*7 and can send alerts via e-mail, RSS or SMS (paid).
Besides these, it has internal monitoring features like keeping an eye on computer/server memory, CPU, etc. with an installable agent. And, has visitor tracking capabilities too.
Mon.itor.us also provides an API for developers who want to embed their applications with the service.