a )
The only contributed module that allows voting on teasers is “Vote up and down“. The limitation of using this module is there is no rating based on 1 -5 or 1- 100. Users can only vote up or down. And the number of votes determines the popularity of the post.

The good thing is using this module you can make your site like digg.com.

b) Another contributed module Tweakbox by user acp is considering to implement this functionality in future version of tweakbox so you can rate on teasers.