Web monitoring service

I want to immediately note that I spent time looking for similar services. Similar services exist, both paid and free.
The main objective of this topic ...
To get the community opinion on monitoring services. Do you use similar services, if so, which ones are paid or free (if you can specify links). What does not suit you in them?

At the moment, the idea of ​​a web resource monitoring service is ripening. The idea is in the “is it worth it?” State.
A small list of features of the service:

• notifications by icq / email / sms;
• customizable reports;
• various monitoring methods;
• api, allowing you to implement your own monitoring algorithms;

ps: please do not kick much - the first topic