Moqups
… is a nifty HTML5 App used to create wireframes, mockups or UI concepts, prototypes depending on how you like to call them.
ClickDummy - Clickable Mockups
Turn your mockups into prototypes and get feedback fast
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 For Desktop
Download the desktop version of Balsamiq. Works offline, one-time purchase.