Mockingbird is an online tool that makes it easy for you to create, link together, preview, and share mockups of your website or application. ~ Mockingbird.
I’ve used Mockingbird, and I can recommend it. Their web app’s minimal approach to design lets you focus on functionality, layout, and user experience without having to nail down what everything is going to look like. This is a great approach to use with clients who have a hard time separating user experience versus design details.