The Dia Diagram Editor is very good Microsoft Visio alternative, it can be used to draw different types of diagrams, includes support for UML static structure diagrams (class diagrams), entity relationship modeling, and network diagrams. It has special objects which let you draw many types of diagrams.
Install Dia Editor in Ubuntu, open the terminal and run the following command.
[root@ubuntu ~]#sudo apt-get install dia
Install Dia Editor in Fedora, open the terminal and run the following command.
[root@fedora16 ~]#yum install dia