One of the most exciting (but not well known and documented) extensibility areas of SSRS 2005 is custom report items (CRI). As its name suggests, custom report items allow you to implement your own reporting controls when the standard SSRS report items (textbox, chart, table, matrix, etc.) are not enough. For example, the attached screenshot shows a report that uses a CRI I wrote to show a KPI value graphically.
If you have experience in developing Windows Forms .NET controls, you will feel immediately at home with CRI. In fact, I dare say that you can take the GDI+ source code of any Windows Forms.NET control and convert it easily to an SSRS CRI. As you can imagine, it won’t be long before third-party vendors come up with all sorts of cool SSRS widgets.
If you want to learn more about CRI, check out the PolygonsCRI sample which Chris Hays wrote. It is included with the latest refresh of the SQL Server 2005 samples. I have an article in the works for MSDN Magazine that explains the implementation details of a progress tracker CRI.. I will post an update here when the article is available (most likely July-August issue of MSDN Magazine).