Moqups is a nifty HTML5 App used to create wireframes, mockups or UI concepts, prototypes depending on how you like to call them.
Collaborative Wireframing and Prototyping for UX Teams.
Mockingbird is an online tool that makes it easy for you to create, link together, preview, and share mockups of your website or application.
Cacoo is a user friendly online drawing tool that allows you to create a variety of diagrams such as site maps, wire frames, UML and network charts. Cacoo can be used free of charge.
The Pencil Project's unique mission is to build a free and opensource tool for making diagrams and GUI prototyping that everyone can use.
Need a diagram, process chart, quick page-layout, website wireframe or graphic design? OmniGraffle can help you make eye-popping graphic documents quickly.
iPlotz allows you to rapidly create clickable, navigable mockups and wireframes for prototyping websites and software applications.
Axure RP is the standard in interactive wireframe software and gives you the power to quickly and easily deliver much more than typical mockup tools.
Online Wireframe Tool - Design, collaborate user interface concepts for your software and websites.