What are some alternatives?
When comparing Akira and Zeplin, you can also consider the following products
Inkscape - Inkscape is a free, open source professional vector graphics editor for Windows, Mac OS X and Linux.
Invision - Prototyping and collaboration for design teams
Sketch - Professional digital design for Mac.
Framer - Bring your designs to life.
Icons8 Lunacy - Free graphic design software with built-in resources. Fully compatible with Sketch and works offline
Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.