Try this one. (yEd) I have not used it much, but it looks pretty good.
http://www.yworks.com/en/products_yed_about.htm
Also here is another one. (Diagram Designer)
http://meesoft.logicnet.dk/
http://www.yworks.com/en/products_yed_about.htm
Also here is another one. (Diagram Designer)
http://meesoft.logicnet.dk/