Headline Split Tester is a new plug-in that allows bloggers to enter an alternate headline for every post on their blog. The headlines are then randomly alternated on the website until a certain number of "headline views" has been reached. At that point, a "winning" headline (as determined by the number of people that have clicked on each headline to date) is determined and that headline is shown going forward.
SES Theme Split Test is a plugin thay enables bloggers to set up two different templates with elements that might increase conversions, serve the different templates up to users, and track their activity using custom segments in Google Analytics.
Visual Website Optimzer is a plugin thay enables bloggers to connect their VWO account in order to create multiple version of your website, define what you want visitor to do, and split test the website traffic amongst those versions. This is a commercial plugin but there is a free trial for evaluation which will test up to 1000 visitors for 30 days (although you can only run one test).