Software Design Tools [Diagrams][Archi & UML]

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Architecture tools:

Archimate [FREE]
http://www.archimatetool.com/download

Lucid Chart [high quality architecture diagrams]
https://www.lucidchart.com/

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UML design tools:

Visual Paradigm [FREE for non-commercial use]
http://www.visual-paradigm.com/download/

Star UML [FREE for non-commercial use]
http://staruml.io/

Enterprise Architect from Sparx Systems
http://www.sparxsystems.com/

Rational System Architect [$$$]
http://www-03.ibm.com/software/products/en/ratisystarch

Confluence & UML
https://marketplace.atlassian.com/apps/41025/plantuml-for-confluence?hosting=server&tab=overview

Umlet - free UML tool (desktop or online)
http://www.umlet.com/umletino/umletino.html

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Sequence diagrams:

Web Sequence Diagrams [FREE]
https://www.websequencediagrams.com/

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General drawing tools: [FREE][OSS]

diagrams.net (previously known as draw.io)
https://github.com/jgraph/drawio-desktop
https://app.diagrams.net/

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References: 

Stack overflow:
http://stackoverflow.com/questions/15376/whats-the-best-uml-diagramming-tool

Wikipedia:
http://en.wikipedia.org/wiki/List_of_Unified_Modeling_Language_tools