Simple Page Tester is a free (for the basic version) and easy to use A/B plugin for your WordPress blog. The user interface is easy to use, but it only allows 2 pages per test.
Nelio AB Testing offers a trial for it’s premium AB testing plugin. This plugin is packed with features including a heatmaps, widget testing, and alternate theme testing.
The Visual Website Optimizer plugin gives you the power of one of the most widely used AB testing platform on the web. Visual Website Optimizer is the most feature rich solution to AB testing.