Nifty - Manage projects, work, and communications in one place.
Invision - Prototyping and collaboration for design teams
Intercom on Product Management - The ultimate resource for aspiring product managers
Proto.io - Create fully-interactive high-fidelity prototypes in minutes that look and work exactly like your app should. Sign up for a free 15-day trial and get started.
productboard - Beautiful and powerful product management.
Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.