There's quite a few. Obviously Visio is popular, on Linux I've used a tool called "Dia" (http://dia-installer.de/ http://dia-installer.de/) there are several o

There's quite a few. Obviously Visio is popular, on Linux I've used a tool called "Dia" (http://dia-installer.de/ http://dia-installer.de/) there are several others, but these are my 2 go to tools for UML.