How to add number do Dock Icon like in Mail?

FROM : Samvel
DATE : Sun Mar 30 16:42:01 2008

I'd like my application to show number of completed tasks in Dock Icon 
as it is done in Mail (number of unread emails). Anyone knows how to 
do that or where I can read about such functionality?

The idea is widely used by next applications:

Adium
Mail
iGetter
Transmission
XCode

They all add some number / text / progress bar to Dock Icon.

Thanks.