My company very recently agreed to open source (under the GPL ), a tool that we use to monitor our web services in real time and to create reports on availability and response time. It is called ServiceMon and can meet your needs.
It runs on Windows as a standalone application and works by following simple script operations that dictate the services that will be monitored. For example, to verify that a web page contains a specific value, similar to webmon, you should use this line:
http-get "http://www.google.com" must-contain "I'm Feeling Lucky"
The frequency with which it executes script operations can be easily configured in the same way as the order in which it processes them.
In addition to monitoring web pages and web services, we use ServiceMon to track the availability statistics of each service and create response time statistics.

ServiceMon is written using the plugin architecture, so you can use .NET to add new types of monitoring operations. So, for example, if your web service uses funk authentication, you can quite easily connect it to the utility.
Full documentation and download instructions here
I hope you find this useful, and I would like to hear your thoughts.
Disclaimer I designed ServiceMon to be a little biased :)
Wheelie
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