Dia - Dia is a GTK+ based diagram creation program for GNU/Linux, MacOS X, Unix, and Windows, and is released under the GPL license.
Zeplin - Collaboration app for UI designers & frontend developers
UMLGraph - UMLGraph is a professional automated drawing tool that allows the designers the declarative specification and drawing of UML class and sequence diagram.
avocode - Centralize design collaboration, version control, prototyping, feedback, and developer hand-off with Avocode. Work on any design, anywhere, with anyone. Sign up free ๐ https://avocode.com/.
draw.io - Online diagramming application
Handoff by Marvel - Automatically turn designs into assets, specs and code