Voting API

VotingAPI helps developers who want to use a standardized API and schema for storing, retrieving, and tabulating votes for Drupal content.

Among other things, it supports:

Rating of any content (comments, nodes, users, fish, whatever)
Multi-criteria voting (rate a game based on video, audio, and replayability)
Automatic tabulation of results (with support for different voting styles, like 'percentage' and '+1/-1')
Efficient caching of results (sorting and filtering doesn't require any recalculation)
Hooks for additional vote calculations

Note that this module does NOT directly expose any voting mechanisms to end users. It's a framework designed to make life easier for other developers, and to standardize voting data for consumption by other modules (like Views).

For some examples of simple voting systems based on VotingAPI, check Is Useful, Fivestar, and Plus1.