I have few ASP.NET sites on different servers. I'm looking for tool to detect and notify if site fail: error 500, asp.net exceptions, validate html by regex. This tool have to be able send requests with specific User Agent in header. As I understand Nagios with module check_http can solve my problem, but I need windows tool with same functionality.
you could write your own check quite easily with WWW::Mechanize; it's Perl, so you can run it in Windows as well.
If you do not want to install nagios, maybe you would like to take a look at opsview community edition, it's nagios with a nice web ui so you can basically manage it from the web ui. They have a vmware image you can use to get you started in no time, and from there you can use check_http