a web service monitoring tool

The most common program in this area is Nagios. It has a scheduler which performs test at intervals you set, it has alarms when something goes wrong and it has plugins which perform the actual test. If there is no plugin for your service (a zillion of plugins already exist), you can write one without touching the main code.

I personally use mon instead because it is much simpler and writing monitors (the small script that do the actual test) is very simple. It does everything you expect from a monitoring tool such as setting intervals for the alarm (no need to tell me every minute that the Web site is down).

JMeter can do it as daemon process, the if you are using unix like platform cron helps you a lot

I'm using http://pingdom.com/ to good effect. It can not only check to see if a website is up or down, but can also be set to check for specific text (eg 'sql error', would be a fail). It's got plenty more features, and very nice reports, as well as regular emails for day-to-day status.

Pingdom has been a paid-for service for a while now, but others, such as https://uptimerobot.com/ are still available, and I use them for low/no-cost, some-frills monitoring.