Take your static designs, wireframes or mockups to the next level. At Proto.io you will find everything you need to create beautiful fully interactive, animated prototypes that tell the full story. via Tour – Proto.io
See the following effect I created to tighten up the layout on my client’s home page:
The above animated GIF (click the graphic for a fullsize view) shows a “click to reveal” button I created using jQuery. The effect is reusable throughout the website’s theme by simply adding .reveal to the parent row.
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.