After having tried a number of testimonial plugins, I came to like GC Testimonials the best. The one issue I (and many others) had with it is the lack of captcha functionality. We found a good day was when we got less than 20 spam testimonials submitted. Instead of waiting for the original author to add captcha we developed our own version for our clients, then decided to share it in the WordPress.org repository.
Original plugin: http://wordpress.org/plugins/gc-testimonials/