The best free alternative to Lucidchart is diagrams.net, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 100 alternatives to Lucidchart and loads of them is free so hopefully you can find a suitable replacement. Other interesting free alternatives to Lucidchart are Dia (Free, Open Source), yEd Graph Editor (Free), LibreOffice - Draw (Free, Open Source) and Freeplane (Free, Open Source).
Clickcharts flowchart software allows for easy drawing of diagrams and flowcharts to provide a way of organising and displaying data. Lay out your ideas, organisation and processes or create UML, Venn or mind map diagrams with this flow diagram maker.
PathVisio is a free open-source pathway analysis and drawing software that allows drawing, editing and analysing biological pathways. It provides a basic set of features for pathway drawing, analysis and visualization. Additional features as plugins.
WhiteStarUML is a fork of StarUML with an intent to revive its Delphi code base by updating code to recent Delphi editions, reducing dependence on third party components and fixing bugs and adding new features.