Please have a look at my answer to Best free UML authoring toolBest free UML authoring tool. It refers Dia, StarUML and argoUML, Visual Paradigm, POPP/POI (Plain Old PowerPoint/Impress), gliffy.com, yEd, Red Koda. Most of them work under Linux.
If they don't, you can make them work under Wine (e.g. Visual Paradigm).