Software :: Use "top" Command To Check The Resource Usage ?

I usually use "top" command to check the resource usage , it is very good and simple method to know the status ( loading , CPU , memory usage ), but we have many servers so we can't check the servers frequently , so we can't find the problem before the problem happens

Any method / script that I can implement to send us a alert mail if the resource usage reach a specific level , for example , if the CPU usage reach 90% , then send us a alert mail.