When working on software design, UI Mockups are quite important and Patrice, colleague of mine at eXo has pointed me to a very cool tool to use when you have to quickly do a mockup, and work with the dev team in an iterative fashion: This tool is "Balsamiq Mockup". Here an example of mockup realized with Balsamiq:
Also if you read more about the tool, you can see that it has native integration with others tools such as JIRA, so cool to be able to integrate a mockup easily when defining a new item...
Take a look to this screencast explaining the basic features and use of the product:
Another interesting tool is the online white board "DabbleBoard" that allows user to quickly create and share graphs, ideas, ... A very cool feature of this tool is the automatic recognition of shapes. The best way to understand, is simply to see it in action: