How to limit number of nodes a user can create per day?
For Drupal 6 I have used Node Limit Number with great success.


  Limit the amount of nodes or comments your users create over a given
  time period. This module has been rewritten to integrate with Rules.
  Instead of going to a page to assign limits you now just need to
  create rules. A default rule has been provided as an example.


The 2.x branch also integrates with the Rules module to make it really flexible.