I just posted the first article in my series 'Building Custom Controls'.  I learned a great deal about the process from this simple control.  Not the least of which was the inherent challenges of building controls that do the bulk of their work at the client.  I also got some good ideas about what kind of control to tackle next.  If you have a suggestion let me know here. 

In the meantime, I am working on a post about adding and testing custom controls.