UMLGraph - UMLGraph is a professional automated drawing tool that allows the designers the declarative specification and drawing of UML class and sequence diagram.
UX Challenges - Done reading about UX? Start doing it.
draw.io - Online diagramming application
uxtoast: Learn UX Design - Become familiar with design laws and learn how to use them.
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.
Uxcel - The easiest way to learn UX/UI design