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.