1. A computer-implemented method of normalizing item recommendation scores, the method comprising: by a computer system comprising computer hardware: assigning scores to candidate recommendations with first and second recommenders, the first recommender configured to assign first scores to a first portion of the candidate recommendations, the second recommender configured to assign second scores to a second portion of the candidate recommendations using a different scoring scale than the first recommender; outputting the scores from each of the recommenders to a normalization engine; for each recommender, normalizing the scores output by the recommender with the normalization engine by at least: combining the scores for at least some of the candidate recommendations to generate a combined score, said combining comprising computing a moving average of at least some of the scores, and calculating normalized scores based at least in part on the combined score and the scores for at least some of the candidate recommendations; and using the normalized scores to select at least a portion of the candidate recommendations to recommend to the target user.