diagrams.net (previously draw.io) is a free online diagram drawing application for workflow, BPM, org charts, UML, ER, network diagrams. No login or registration are required and features include the ability to save locally (including SVG), a range of stencils, .
diagrams.net vs OmniGraffle opinions
Pencil is a standalone app that lets you create diagrams or UI mockups. It doesn't have collaboration/multi-user features.Discontinued
The last release/commit was in October 2019.
Pencil Project vs OmniGraffle opinions
yEd is a powerful diagram editor that can be used to quickly create diagrams manually or import external data for analysis and auto-magically arrange even large data sets by just pressing a button. You can use yEd to quickly create flow charts, BPMN and UML diagrams...
yEd Graph Editor vs OmniGraffle opinions
Draw lets you produce anything from a quick sketch to a complex plan, and gives you the means to communicate with graphics and diagrams. With a maximum page size of 300cm by 300cm, Draw is a an excellent package for producing technical drawings, general posters, and much else.
Dia is a GTK+ based diagram creation program for Linux, Unix, Mac OS X and Windows released under the GPL license.
XMind is a brainstorming and mind mapping application. It provides a rich set of different visualization styles, and allows sharing of created mind maps via their website.
XMind vs OmniGraffle opinions
Doesn't serve the same purpose. It's mindmapping app.
Xmind only does mind maps. Omnigraffle does many other types of diagram.
PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams. The generator can be run anywhere within JVM and integrated with various application such as wiki, text editor, IDE, programming language, documentation generator, and others.
Adobe XD (Experience Design) is a vector-based tool developed and published by Adobe Inc for designing and prototyping user experience for web and mobile apps.
Graphviz is open source graph visualization software. Graph visualization is a way of representing structural information as diagrams of abstract graphs and networks. It has important applications in networking, bioinformatics, software engineering, database and web design...