UI

Balsamiq Mockups
a rapid wireframing tool that helps you Work Faster & Smarter. It reproduces the experience of sketching on a whiteboard, but using a computer. Making mockups is fast. You’ll generate more ideas, so you can throw out the bad ones and discover the best solutions.
Desktop: $79; Web Subscription: $12/mo.
http://balsamiq.com/

InVision
Quickly transform your designs into beautiful, fully interactive prototypes complete with gestures, transitions & animations for web, iOS & Android.
Free for a single project. Paid accounts start at $15/mo.
http://www.invisionapp.com/

UXPin
Inspired UX Design is tough with uninspired user experience design tools. That’s why UXPin is handcrafted to be robust and elegant.
Starting at $15/mo.
https://www.uxpin.com/

Mockingbird
An online tool that makes it easy for you to create, link together, preview, and share mockups of your websit or application. More polished appearance than Balsamiq’s rough sketch.
Starts at $9/mo for two active projects.
https://gomockingbird.com/

MockFlow WireframePro
A web-based tool to design and collaborate user interface blueprints for websites and apps. Its helps to visualize the website’s interface, navigation and structure in short time.
Free for one project. Starts at $14/mo. for paid accounts.
http://mockflow.com/

OmniGraffle
For creating precise, beautiful graphics. Like website wireframes, an electrical system design, a family tree, or mapping out software classes. For artists, designers, casual data-mappers, and everyone in-between.
Mac starting at $99.99. iPad $49.99.
http://www.omnigroup.com/omniGraffle/

Gliffy
Create professional-quality flowcharts, Org charts, UML diagrams, Network diagrams,
Wireframes, Technical drawings and more. Gliffy works directly in your browser!
Free.
https://www.gliffy.com/

Pen and Paper
Yes. An actual pen, and some real made-from-wood paper. It’s often fast and easy to get your ideas down on paper first and refine them in software later. :-)