ASP.NET Ajax UpdatePanelNotifier: get notified of partial updates
UPDATE: The control has been added new features, check it out here.
The control exposes the same properties as the standard UpdateProgress control beyond two new properties which let you enter the name of the functions you want to execute:
Note that the bound functions will be called only when the partial refresh is triggered by a control inside the UpdatePanel and not when it's an external trigger doing it, since in this case it's not yet possible to retrieve the Id(s) of the UpdatePanel(s) which will be refreshed in response to that action. Anyhow it seems that the ASP.NET Ajax Team is working on it and will eventually provide this facility in the final release.
You can watch a short screencast to get started fast. The source code is available on my main project's svn repository.