Dia is free and open source drawing software.

Microsoft Visio (formerly Microsoft Office Visio) is a diagramming and vector graphics application and is part of the Microsoft Office family.

Diagram Designer is a vector graphics editor that allows you to create UML class diagrams, illustrations, flowcharts and slide shows.

Grafio is a fun and capable diagraming app for iOS with plenty of features at a fraction of the cost of others.

Cacoo is a user friendly online drawing tool. Multiple users editing the same diagram means Real-Time Collaboration.

Nevron Draw is a software for creating flowcharts and diagrams.

VisiTouch helps you view and convert to PDF your MS Visio diagrams for iPhone.

Pencil Project is an open-source GUI prototyping tool.

Google Drawings is a free, web-based diagramming software developed by Google.

draw.io is free online diagram software for making flowcharts, process diagrams, etc

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

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

Storyboard That is a free online storyboard creator.