https://umbrello.kde.org/
Umbrello UML Modeller is a Unified Modelling Language (UML) diagram program based on KDE Technology.
UML allows you to create diagrams of software and other systems in a standard format to document or design the structure of your programs.
You may take a look at the screenshots to see umbrello in action.
Our handbook gives a good introduction to Umbrello and UML modelling.
Umbrello comes with KDE SC, included with every Linux distribution and available through your package manager. See Installation to install Umbrello.
For user support join the Umbrello mailing list.
Umbrello is part of KDE Software Compilation which means it comes with all GNU/Linux distributions. You will be able to install it using your normal software installation app (Muon, Software Centre, Yast etc) or command line package manager (yum, up2date, yast, zypper, apt-get, emerge etc).
The package will be called umbrello, some older distros will package it as kdesdk.
At least on fedora 20 it is also required to install the package oxygen-icon-theme, otherwise umbrello will not show any icons.