Dia is free and open source drawing software.

UMLet is an open-source Java-based UML tool designed for teaching the Unified Modeling Language and for quickly creating UML diagrams.

Storyboard That is a free online storyboard creator.

Pencil Project is an open-source GUI prototyping tool.

Visual Paradigm (VP-UML) is a UML CASE Tool.

Nevron Draw is a software for creating flowcharts and diagrams.