If there is enough interes in this kind of activity, there is a need for a
couple of volunteers to organize everything and to work as judges on the
competition days (or weeks). They should decide, what the competition
is about etc. What do you think? Pros & Cons? Would you take part
in this kind of competition(s), if these were organized?

I don't think these kinds of competitions work in open source development. Especially not with such a small community as Haskell's. The beauty of open source development is the cooperation between programmers that share their code and discuss ideas with others. All that disappears when you start a competition because sharing code and discussing ideas might help others rather than helping yourself. Read the following article for further information:
http://www.osx86project.org/index.php?option=com_content&task=view&id=127&Itemid=2

But that being said, I still applaude your initiative and the fact that you're trying to come up with some concrete and constructive ideas as to how we might get some more libraries. We do indeed need more libraries. I think one of our problems here is that people somehow set to bar too high. I get the feeling that people are afraid of showing their code (I know I am) because they think that it must be very well-polished and have a very well-thought-though design. It's like everything Haskell needs to be elegant and beautiful as a mathematical theorem. But it will probably do us good to allow for some hackish libraries. They can always be improved upon later.

Cheers,

/Josef