To write your own Extender Control, I am using Microsoft Ajax Library.
This location below has a powerpoint presentation and a zip with a demo code.
The power point presentation talks some basics about Microsoft Ajax Library key components and the demo shows how to make an extender control.
Please remember to select extender control template while creating extender control in Visual studio. It will save you lot of configuration lines that you might have to write otherwise.
https://docs.google.com/leaf?id=0B_ZpzrhCCVOoNTE0N2ZkMDktNWQ5Yy00ZTIwLTg4ZTAtY2Y5YjE0ODEyYTYy&hl=en&authkey=CIqq8CQ
