Are there any good + free + Lightweight+ Linux UML Design tools?

Did you mean a tool to draw the diagram? Try Dia Diagram Editor.

Most of the UML tools for Linux look really bad, but you could use these:

• ArgoUML (Java based, I had some glitches with diagrams)
• Umbrello
• Violet UML Editor (Java required)
• UMLet (Java required)
• Dia
• StarUML

Experimental/alternatives:

• ASCII flow - you can not always include images in documentation but simple text looks everywhere same, good to use in code for quick overview.

• Cloudcraft - Draw AWS diagrams with Cloudcraft. Good for quick architecture diagrams.

For quick drawings and who does not scare to "code" some diagrams I also suggest to take look at this product http://yuml.me/diagram/scruffy/class/samples.

I just discovered Violet UML. Easy to use, lightweight, diagrams look ok for me: I picked it for my use!

See here : http://alexdp.free.fr/violetumleditor/page.php

i like UMLet: http://www.umlet.com/. It's very light weight