Tool for ongoing monitoring of a webserver?

I need to monitor a SaaS by going to a specific URL (is currently done either manually in a browser or by sending an HTTP request programatically in C# ) and if the response is other than HTTP response code 200, send an email and/or sms to predefined list of addresses/phone numbers.
We'd like to be able to specify the number of the retries after which no request will be send for specified amount of time, and also have the tool not send any more alerts after sending a predefined number of them.

Is there an existing tool available that can perform the above?

Thanks!

From serverfault akapulko2020

• Nagios is my tool of choice for this sort of thing. Have a look here. It probably looks like overkill, but it certainly does exactly what you want. We have it running for about 200 devices (servers, printers, router, switches) and about 800 monitored services.

  If you have detail questions about configuration, just post them here and I'll be glad to answer them.

  Matt : +1 for Nagios. My current Nagios setup monitors 200+ hosts, 700+ services. If Nagios is more than you need, I'd bet you could script up something with wget or curl.

  brent : Another + for Nagios. You can configure people and groups of people, assign them to different monitoring alert periods (e.g., after hours support crews get alerts while 9-5ers don't). We also have it setup to send SMS warnings through a GPRS modem.
  From wolfgangsz

• Pingdom has a low cost service providing exactly what you describe. You can monitor a single server for free. 5 servers is $9.95 per month.

  From MarshallY

• Zabbix has a web monitoring feature that provide what you needs. Hope this helps.

  From Maxwell

• Nagios is your best choice for onsite monitoring on Linux. For hosted ("cloud") monitoring we like AlertFox. Unlike other services, it uses real web browser for transaction monitoring. I find that very useful. It is something that Nagios can not do out of the box so it complemnts our internal Nagios setup.

  If you are on a tight budget, AlertFox has a free account option, but it is a bit hidden (link here) ;-)

  From MFauser