The process for creating your own module follows a few simple steps.
copy the skeleton information to a newly named directory
modify the skeleton to reflect the name of your module
create .inc files that display the elements on the screen
create any php files needed to fill in forms
The process is fairly straight forward and after learning how the Horde API' s work, you should be able to produce new modules in a minimum amount of time.
Jan Schneider talked about this at FOSDEM 2005. You can find his slides at http://www.horde.org/papers/fosdem2005/.