Collection of functions to add karma functionality to your wordpress. So when a post is being voted up, the karma of the author will be up. Just like reddit. This is very basic. Fell free to use and modify.

karma_helpers_for_wp.php

<?php

/* 
to begin, please create a table named [prefix]votelog e.g. wp_votelog
with the following three fields:
id : auto_increment, BIGINT, primary_key
voter_id: BIGINT
post_name: TEXT
*/

/* echo the score of a post */
function the_vote_score($echo = true){
    global $post;
  $score = get_post_meta( $post->ID, '_vote_score', true );
	if ($score == '') {
	    add_post_meta($post->ID, '_vote_score', 0);
		$score = 0;
	}
	if ($score > 0) {
	    $score = (string)$score;
		$score = '+'.$score;
	} else {
	    $score = (string)$score;
	}
	if ( $echo === true )
		echo $score;
	else
		return $score;
}

/* 
vote a post up. use inside a loop 
to vote down, use -1
*/
function voteup($up_by=1){
  if ( is_user_logged_in() ) {
    global $post, $wpdb;
  	// already vote?
  	$user_ID = get_current_user_id();
  	$is_voted = $wpdb->get_row("SELECT * FROM $wpdb->votelog WHERE voter_id = ".$user_ID." AND post_name = '".$post->post_name."'");
  	if (count($is_voted) < 1) {
  		$score = get_post_meta( $post->ID, '_vote_score', true );
  		if ($score == '') {
  			add_post_meta($post->ID, '_vote_score', 0);
  			$score = 0;
  		}
  		$score = (float)$score;
  		$score = $score + $up_by;
  		
  		update_post_meta($post->ID, '_vote_score', $score);
  		$wpdb->query(
  			"
  			INSERT INTO $wpdb->votelog 
  			(voter_id,post_name)
  			VALUES (".$user_ID.",'".$post->post_name."')
  			"
  		);
  		// update author karma
  		$author = $post->post_author;
  		$karma = get_user_meta( $author, '_author_karma', true );
  		if ($karma == '') {
  			add_user_meta( $author, '_author_karma', 0);
  			$karma = 0;
  		}
  		$karma = $karma + $up_by;
  		update_user_meta( $author, '_author_karma', $karma );
  		return true;
      exit();
  	}
  }
  return false;
}

/* check if the current user already voted the post */
function has_user_voted(){
  if ( is_user_logged_in() ) {
    global $post, $wpdb;
    $user_ID = get_current_user_id();
	  $is_voted = $wpdb->get_row("SELECT * FROM $wpdb->votelog WHERE voter_id = ".$user_ID." AND post_name = '".$post->post_name."'");
  	if (count($is_voted) > 0) {
  	    return true;
        exit;
  	}
	}
  return false;
}

/**
 * Retrieve a user's karma.
 *
 * @param	int|object	User ID, or the entire WP_User object
 * @return	int			the author's karma score
 */
function author_karma($user) {
	if ( is_object( $user ) )
		$user = $user->ID;
	$karma = get_user_meta( $user, '_author_karma', true );
	if ($karma == '') {
	    add_user_meta( $user, '_author_karma', 0);
		$karma = 0;
	}
	return $karma;
}

/* run this if you really need to */
function fix_all_author_karma() {
    global $post;
	$author = $post->post_author;
	if ( !$author ) return;
	
	$meta = get_user_meta( $author, '_author_karma', true );
	if ($meta == '') {
	    add_user_meta( $author, '_author_karma', 0);
	}
	
	$new_karma = 0;
	$author_posts = get_posts( array( 'author' => $author, 'numberposts' => -1 ) );
	foreach ( $author_posts as $p ){
	    $post_meta = get_post_meta( $p->ID, '_vote_score', true );
		if ($post_meta != '') {
		    $new_karma += get_post_meta( $p->ID, '_vote_score', true );
		}
	}
	
	update_user_meta( $author, '_author_karma', $new_karma );
}