You could try any of these modules (with quotes included from their project pages):
- https://drupal.org/project/fivestar or
... adds a clean, attractive voting widget to nodes and comments and any entity.
- https://drupal.org/project/rate or perhaps
... provides flexible voting widgets for nodes and comments ("The cck model for voting with Drupal").
... allows votes on some Drupal entities and provides the base for implementing votes on other entities.