InvisionApp: Does more than simply prototyping online, can track versions, incorporates kanban board for workflow management, has comments, freehand, and is always moving forward with more! Obviously most familiar with this one, but there are others stated below.
Digital Product Design, Workflow & Collaboration | InVision
Proto.io - Prototypes that feel real
Origami by Facebook
Origami Studio - Design Prototyping
Principle: primarily for intricate animations (screen to screen, button effects)
Avocado by IDEO: UX prototyping library built on Facebook's Origami. Has preset patches to create prototypes without writing a single line of code.
Figma: collaborative design tool
Figma: the collaborative interface design tool.
Flinto