KingSumo

KingSumo is a popular wordpress plugin that creates a standalone page on your blog (it can also be installed by itself in a wordpress directory if you have an otherwise static site). It’s somewhat limited in features, but the interesting thing about KingSumo is that it encourages sharing the contest with others, because the participant gets extra entries for every person that they get to also sign up. Which means the more people they bring in, the better their chances of winning, and hence the encouragement of virality for your competition.