Dia Alternatives

yEd is a free desktop application to quickly create, import, edit, and automatically arrange diagrams. It runs on Windows, Mac OS X, and Unix/Linux.

Online diagramming application

LucidChart is the missing link in online productivity suites. LucidChart allows users to create, collaborate on, and publish attractive flowcharts and other diagrams from a web browser.

PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.

Simplify and communicate complex information using data-linked diagrams.

StarUML

OmniGraffle is for creating precise graphics like website wireframes, an electrical system designs, or mapping out software class.

Draw, LibreOffice, ODF, open standards, creative software, vector drawing

UMLGraph is a professional automated drawing tool that allows the designers the declarative specification and drawing of UML class and sequence diagram.

UMLet - Free UML Tool for Fast UML Diagrams

ArgoUML, free and safe download. ArgoUML latest version: Open-source diagramming application for personal computers. ArgoUML is a program that allows users to create complex and professional diagrams.

Draw.io is a free and open source web based diagramming and mind mapping software.

Umbrello UML Modeller is a Unified Modelling Language diagram programme for KDE.