Page Flows - User flow design inspiration for mobile & desktop
UX Flow Wireframe Prototyping System - Create beautiful user flows prototypes for your next project
Connection Flow Arrows - Create connected arrows for sitemaps & user flows in Sketch
draw.io - Online diagramming application
Flowkit - Sketch library for user flows/content maps/annotations
yEd - yEd is a free desktop application to quickly create, import, edit, and automatically arrange diagrams. It runs on Windows, Mac OS X, and Unix/Linux.