This fantastic walkthrough is for developers who need to build and deploy a server object extension for use in server applications. It describes the process of extending the MapServer to provide methods for performing a specific type of spatial analysis on the features in one layer of the map.  The scenario consists of four parts:

• Developing the server object extension
• Developing the ArcCatalog and ArcGIS Server Manager administration property pages for the server object extension
• Registering the custom server object extension  
• Developing a client application to work the server object extension

Read the whole complete tutorial here:

http://resources.esri.com/help/9.3/arcgisserver/adf/dotnet/developer/scenarios/extend_svr_object.htm

If you are thinking in extending ArcGIS Server, this is the best documentation I found that got me running.

You can actually get the sample here.

Even if the link says 9.3 it works in 9.3.1.

Cheers

Al

Related links.

Extending a ArcGIS Server geocoder.

Creating a Map Service Browser