Umbrello UML Modeller is a Unified Modelling Language diagram tool for KDE. It is able to produce all types of UML diagrams. It imports C++, and it generates code in 10 languages. It uses an XMI-based file format. It is part of KDE, with sources and packages available for those using older versions of KDE.
no real UML tools?
"there are no real UML development tools under
Linux" says their website.
(Possible bias involved.)