Axure - The most powerful way to plan, prototype and hand off to developers, all without code. Download a free trial and see why professionals choose Axure RP 9.
Really Good UX - A library of screenshots for UX inspiration
Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.
Usability.gov - Resource for user experience (UX) best practices & guides
Moqups - The most stunning HTML5 app for creating resolution-independent SVG mockups, wireframes & interactive prototypes for your next project
UX Check - Easy heuristic evaluations on your website (chrome ext.)