A key part of interface design is an exercise known as "wireframing." In this design phase, elements of an interface are blocked out roughly to show relative placement, interaction, and functionality. It is a rapid way to talk through functional requirements of a project and get buy-in from stakeholders without having to waste a lot of time on visual design that won't remain in a finished product.

Mockflow is a flash-based, online, collaborative wireframing tool for Web and Software designers. It contains a fairly complete set of wireframe elements and icons for use in your wireframe with flexible customizability of all the elements. The killer app of the tool is it's ability to collaborate in real-time with other team members online. Very, very useful for distributed teams.

I tend to use Adobe Fireworks for all my wireframing (and everything else) but a coworker pointed me toward this tool and it captured my attention. I find flash-based tools distasteful, they always feel slow to me, but this one was simple enough, with enough features to make it compelling to use. Definitely the right tool for the right situation.

The basic version is free, but ad supported and you are limited to two collaborators. Upgrading (introductory price of $49 / year) grants you unlimited collaborators and projects, is ad-free, and gives you 500 MB of storage. Definitely worth a look for distributed Web teams.