There's no point executing a bunch of campaigns to get traffic to your site if your site is down when people arrive. If your site is down now how would you know ?, if you're not sure then you need a website monitoring service. What services do you use for this purpese?

I think that it is integral component. There is a lot of different services. And I have already tried such services as site24x7 mon.itor.us and wathmouse But they have a number of different defects. I prefer Dotcom-monitor it is cheap, smart and has a lot of features

You can use following site.
http://www.siteuptime.com