As I am moving away from use of microsoft softwares, I was trying to find a proper alternative for visio, the famous microsoft modeling tool.
There seem to be 2 of them as proper alternatives(dia and drawing of open office), but dia leads the way because of the similarity with visio and certain special objects which make it better in casual designing. http://projects.gnome.org/dia/.
Check the URL for more details. It is a wondeful contribution by Free Software Foundation.