We need mockups. They allow us to communicate with developers, clients, users and everybody in between in a common language. I'm a big fan of the Sharpie and a pad of paper. Balsamiq Mockups has created a really great piece of software to speed up the mockup process.

I'm also a big fan of mind maps. Using Balsamiq feels a lot like mind mapping, but with UI structures. It opens the door for new workflows. For me a new project will now consist of the mind map -> Balsamiq generated mockups -> refined wireframes -> implementation.

Balsamiq allows you to save your mockups, both in an application specific XML format (BMML) and as png images. The png files are great for printing scratch sheets from templates that you have mocked up. These are extremely useful for meetings and annotation with the trusty pen.

One thing I really hope they consider for future versions is some sort of API for extending the available widgets. it would be great to build on the mockupstogo community with lower-level user generated components. The ability to share fully mocked components is awesome, but I always want more control.

(edit) I thought that I would add that after playing a bit more I found that you can import your own 'widgets' using the Image object. It also lets you sketch an image on the fly, effectively allowing unlimited custom objects. Sweet.