Weld - Create websites with any design, for any device, for free, including domain.
Marvel - Turn sketches, mockups and designs into web, iPhone, iOS, Android and Apple Watch app prototypes.
Invision - Prototyping and collaboration for design teams
Icons8 Lunacy - Free graphic design software with built-in resources. Fully compatible with Sketch and works offline
Flinto - The prototyping tool designers have been waiting for.
UXpin - Design is really about solving problems. UXPin is the UX Design Platform that gets that right.