Web resource monitoring service

I want to immediately note that I spent the time looking for similar services. Similar services exist, both paid and free.
The main task of this topic ...
Learn community opinion about monitoring services. Do you use similar services, if yes which one - 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 growing. The idea is in the state of "is it worth it?".
A small list of service features:

• icq / email / sms notifications;
• custom reports;
• various monitoring methods;
• api, which allows you to implement your own monitoring algorithms;

ps: please do not kick much - first topic