http://uptimerobot.com is service that can monitor if you sites are up and running. I have been using it for more than 3 years and it works very well.
The service is free and you can add up to 50 monitors. It is easy to configure a monitor. You don’t have many options, but it can do just what I need.

You can create 4 types of monitors.

• http(s): that’s perfect for website monitoring. The service regularly sends requests (which are the same as if a visitor is browsing your website) to the URL and decides if it is up or down depending on the HTTP statuses returned from the website (200-success, 404-not found, etc.)
• ping: this is good for monitoring a server. Ping (ICMP) requests are sent and up/down status is decided according to the “if responses are received or not”. Ping is not a good fit for monitoring websites as a website (its IP) can respond to ping requests while it is down (which means that the site is down but the server hosting the site is up)
• keyword: checks if a keyword exists or not exists in a web page
• port: good for monitoring services like smtp, dns, pop as all these services run from a specific port and Uptime Robot decides their statuses if they respond to the requests or not.

You can get alarms by 4 different channels.

If you site is only available for certain public IPs, you can see the source IPs in the faq.

You can also use the service in your own applications via an API.

Until now it has been a hobby project for the people running the service, but they have just announced that they now are working on it full time.

While it is nice that the service is free, it would be good with a professional option. Hopefully that will happen at some point.

You can follow them on twitter and facebook.