Moqups
… is a nifty HTML5 App used to create wireframes, mockups or UI concepts, prototypes depending on how you like to call them.
iPlotz
… wireframing, mockups and prototyping for websites and applications
Mockingbird
Wireframes on the fly … an online tool that makes it easy for you to create, link together, preview, and share mockups of your website or application.
Mockups To Go
A user-contributed collection of ready-to-use UI components and design patterns built using Balsamiq Mockups.
Mockups For Desktop
Download the desktop version of Balsamiq. Works offline, one-time purchase.