Sometimes you may want to do a quick OM call without actually writing an application.
This can on occasion be done using the existing web services. Not every call allows for it--but many do, and you might just find yourself in a situation where you can use this.
Here's what I'm talking about. If you go to http://server/_vti_bin/UserGroup.asmx, you'll find the available operations you can perform from this web service. Click on GetUserCollectionFromWeb. The default install does not allow you to post, and simply shows you sample output. To change this behavior, go to \Program Files\Common Files\Microsoft Shared\web server extensions\60\ISAPI and edit the Web.config file that is there. You will see it as follows:
<remove name="HttpGet" />
<remove name="HttpPost" />
<remove name="HttpPostLocalhost" />
<add name="Documentation" />
<trust level="Full" originUrl="" />
Change the lines that begin with <remove so that they begin with <add and then go back to your ASMX page via IE. Now go back to GetUserCollectionFromWeb and you will see an invoke button.
You will see the XML response of the OM calling the related OM function. This can be a useful way to gather or even change items within SharePoint. Some web services have textbox forms for you to enter information to add or change existing objects.