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.
Startup Timelines - How the frontpages of famous startups changed over time
Moqups - The most stunning HTML5 app for creating resolution-independent SVG mockups, wireframes & interactive prototypes for your next project
UXTesting - Discover how your mobile users feel