From their site:
If you are a business with an online presence, it is important that your website and other online services are available 24/7, all year round. However, all websites have occasional problems, whether the reason is external or internal, and there is a significant risk that you are unaware of these problems. Just because the company website is accessible from inside the company, does not necessarily mean it available from the Internet.
Poor online availability can become costly both in terms of lost goodwill and lost business opportunities. The Internet is more and more becoming the main point of contact, and if you are not there to be seen, you become invisible.
With Pingdom, any problem with your website will be discovered the minute it happens. You will be immediately alerted so you can take action before it affects your business. You will also be able to see detailed statistics of your website performance, giving you a historical record of all incidents.
And in terms of features?
Get alerts right away when your site has issues
You may already have internal tools for monitoring your infrastructure, but don’t forget about the end-user experience. People access your website from the outside, from the Internet. Pingdom does this external monitoring for you, making sure your website is reachable and responding well. You can get alerts both via email and SMS when there is a problem.
Track your uptime and performance
With Pingdom it becomes easy to see how much downtime your site has (and when) and how responsive it is. Charts and tables show your uptime and performance over time, so it becomes easy to spot trends and potential problems.
Get help with troubleshooting
Whenever Pingdom detects an error, additional tests are automatically performed to help you with your troubleshooting.
Monitor more than just your website
With Pingdom you can monitor websites, mail servers, DNS servers, FTP servers, networks, and much more.
Related Link: http://www.pingdom.com