What are some alternatives?
When comparing Doorbell.io and Invision, you can also consider the following products
UserSnap - Usersnap Track is your central place to organize user feedback and collect bug reports. One screenshot is all it takes. Get rid of endless email exchanges and coffee-stained sticky notes. Your new life is awaiting you!
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.
UserVoice - UserVoice integrates easy-to-use feedback, helpdesk, and knowledge base management tools in one platform that empowers users to speak and companies to understand.
Moqups - The most stunning HTML5 app for creating resolution-independent SVG mockups, wireframes & interactive prototypes for your next project
UseResponse - Open-source, self-hosted customer feedback software, live chat and helpdesk system that you can install on your server. Organize documentation using knowledge base and get feedback from social networks with centralized system
Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.