Apps with 'Diagrams' feature
diagrams.net (previously draw.io) is a free online diagram drawing application for workflow, BPM, org charts, UML, ER, network diagrams.
Pencil is a standalone app or Firefox extension* that lets you create diagrams or UI mockups. It doesn't have collaboration/multi-user features.
Dia is a GTK+ based diagram creation program for Linux, Unix, Mac OS X and Windows released under the GPL license.
Diagramming and Flowchart making software that is part of the Microsoft Office Suite.
Google Drawings allows users to collaborate creating, sharing and editing images or drawings.
An easy tool for generating diagrams and flowcharts from text in a similar manner as markdown.
Visual Paradigm for UML (VP-UML) is a UML design tool and UML CASE tool designed to aid software development.
Whimsical is a collaborative visual workspace for creating flowcharts, wireframes, virtual sticky notes, and more!.
Venn diagrams, mind maps, take notes, sketches and do other diagrams. Everything is custom and modifiable.
UMLet is an open-source UML tool with a simple user interface: draw UML diagrams fast, export diagrams to eps, pdf, jpg, svg, and clipboard, share diagrams using Eclipse, and...
Simple web based ascii flow diagram drawing tool, draw boxes, lines, arrows and type, then export straight to text/html.
Storyboard That is the world's best online storyboard creator. Everyday people from all over the world use Storyboard That to quickly create amazing storyboards.Teachers
Cacoo is a user-friendly online diagramming tool that allows you to create a variety of diagrams such as wireframes, flowcharts, UML diagrams, network diagrams, mindmaps...
Simple vector graphics editor for creating flowcharts, UML class diagrams, illustrations and slide shows.
Monodraw allows you to easily create text-based art (like diagrams, layouts, flow charts) and visually represent algorithms, data structures, binary formats and more. Because it's all just text, it can be easily embedded almost anywhere.