I used a software named GraphThing. Check it out if it works for your problem

Does this work on windows 7....

you use it on windows using cygwin I am not a windows user

Check out one of these applications, they may be fit your needs. Both of them are Windows portable freeware:

http://portableapps.com/apps/office/dia_portable

http://portableapps.com/apps/graphics_pictures/pencil-project-portable

You may find useful other applications form portableapps. Freemat it's a basic level Matlab Clone (portable also). It doesn't have Simulink, but works with tensors.

Good luck.

Thank You : Mr Abdullah Yousafzai & Mr Teodor Petrita

Definitely yEd (http://www.yworks.com/en/products_yed_about.html). Although it is focused on graphs I use it for diagrams in general. It is written in Java so it works in all platforms.

@: Mr. Alberto Ballesteros

Indeed this one is really good.

Excellent ease of use !

Thank You so much.

Regards: Majid

If you need to generate graphs automatically then GraphViz is very good:

http://www.graphviz.org/

Try netdraw