Today I finally found something that seems to be a tool that I was looking for. The "Violet UML Editor" is amazingly simple, portable and easy to use. It is well organized and even good looking. I was able to create simple and even more complex diagrams without having to think about return types and all other obsolete stuff (a least during architectural design phase). The fact that it is distributed as a single jar file is also a big plus - no installation required no matter what the operating system you work on.
Check it out: http://alexdp.free.fr/violetumleditor/page.php
