Below are some free-licensed and opensource tools can be used for diagram designs, workflow designs, flowchart design, process design and many more.
yEd Graph Editor
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, organization charts, mind maps, and many other kinds of diagrams, graphs, and networks. Choose from a wide range of highly sophisticated layout algorithms to automatically arrange diagrams in no time. An intuitive and visually appealing user interface makes creating diagrams fun. Once a diagram has been created, it’s easy to save, print, or export it to popular formats like PDF, SWF, EMF, SVG, JPEG, GIF, PNG, or HTML image maps.
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, Visio import and real time diagram sharing.
Pencil is a standalone app or Firefox extension that lets you create diagrams or UI mockups. It doesn’t have collaboration/multi-user features. The original developers seem to have abandoned the project since since November 2013; Subsequently the project has been forked and development now continues on github.
ProcessMaker is a cost effective and easy to use open source business process management (BPM) or workflow software application. Workflow software such as ProcessMaker can assist organizations of any size with designing, automating and deploying business processes or workflows of various kinds.
Are you using an opensource tool and want to be added in the list? please add it in comments and I will update the post.