I should say that calibre has a very active development community, with lots of contributions from third party developers. As well as a very active user community as evidenced by these forums. I don't develop calibre for the money, though donations do allow me to spend a larger fraction of my time on it.
For an incomplete list of the contributions to calibre, see
http://calibre.kovidgoyal.net/wiki/D...t#Contributors
Also I note that for calibre 0.7, calibre will become database backend independent and plugin based. At that time it will be considerably easier to implement this (and a lot of more important) features. I'm not saying I will implement it then, but it should become trivially easy for some one who wants to, to write a plugin.
EDIT: Also for a good feel of how much of calibre's functionality has been coded by people other than me, just look at Preferences->Plugins to see how many people have contributed plugins. Also I think about 70% of calibre's recipes have been contributed by users.