Friends could tell me free uml tool for using? I use Argo UML but I don't like it.
Try StarUML, which is open source, quite similar to IBM rational rose, pretty good and small.
I personally use Dia. It's a great drawing application not just for UML, but also flowcharts and gazillion of other models. It has also portable version.
I use Dia - http://projects.gnome.org/dia/
It's pretty much like Visio but free and quicker to draw diagrams with.
More UML tools references (including links to free and open source tools): http://modeling-languages.com/content/uml-tools
Before I used Poseidon for UML, StarUML, Violet UML Tool. Those tools are also not bad, but when creating complex sequence diagram I faced a problems with them and could not manage to solve the problem. I tried Dia - http://projects.gnome.org/dia/ and could manage to create my complex sequence diagram. From now I will try to use only Dia for creating UMLs.
As nobody mentioned it, if you are interested in the diagrams for communication and not in using the model for code generation http://www.umlet.com/ is great, small and you can get as formal as you want.
Another favorite is http://www.websequencediagrams.com/
See http://www.reversejava.com for a dynamic reverse engineering application which generates UML Sequence diagram and view of Participating Class diagram from any Java Application at runtime All you have to do is just run your application and sit back. Reverse Java runs in background tracing all the activities happening inside your application and creates UML diagram for you.
You also have options for, excluding packages,editing the Sequence diagram and exporting the diagrams as PDF or Image.
And its not expensive !
In a days I tried out NetBeans UML tool plugin and happy with it. It is free and right in the IDE.
Have you tried UMLet? This is the easiest one I found. I tested some tools in recent years and this one is the fastest and most intuitive for me.
I don't know your requirements, but you should give IBM Rational Modeler a try. It has a free version, if you want more features, you can pay for the corporate version.