Here is one of the best online available tool for UI Mockups
Click on the containers tab, and drag a new dialog/window onto the page. Click the dialog/window and then look at the little pop-up on the right. You can trivially add a scrollbar and indicate where the thumb should be. You can turn on/off the icons in the top right corner. Adjust the status bar region.
Its very user friendly and very easy to use .. we folks can have hand on this…
We need to download this for use…
here is the print screen of application
Interesting point is even Microsoft Expression team is coming up with SketchFlow in its coming BLEND 3. This will be great for all …
here u go for detail information Click Here
In addition to the SketchFlow and the infamous Balsamiq, I would also like to add another nifty tool called FlairBuilder.
Its good and impressive tool, please have a demo walkthrough. Click here