<p>we need reports in our web application and there is the free ReportViewer Control from microsoft (normally used, to display reports from the reporting services). I like the fact, that the Report Format (.RDL-Format) from the ReportViewer is a documented XML-Format. But the functionality is somewhat limited, when the ReportViewer is used without the ReportingServices.</p>

<p>Is there a good replacement, which is based or compatible with the .RDL-Format?</p>

<p>The first Reports are build in the application, but later, the customer should make his reports by himself.</p>

<p><strong>The Application is a ASP.NET Web-Application</strong></p>

This depend on your requirements. I am not clear what your requirements are.
Is your application a web application or a desktop application?
If your application is a
web application
then you can use any other reporting service. I like
i-net Clear Reports
. There is also a free and fully functional GUI
report designer
that is easy to use. Your customer can create your own reports.
If you have a
desktop application
then you are limit to the language of your application.
You should also think about the platforms. The reporting services are limited to windows and a SQL Server is needed. Does all your customer have a SQL Server?