Quince is a UI/UX/IxD pattern explorer that gives designers and developers new and better ways to discover and find design patterns that can help them make better design decisions and more easily communicate the rationale and implementation details for the choices they make. Quince Pro is a premium design tool/service that enhances and builds upon what folks get for free in the community version.
Quince Pro leverages the community/public patterns (if you want to use them) while giving designers tools specifically tailored at organizing and communicating interaction and visual design frameworks. By providing a cloud-based solution, Quince Pro lets designers carve out private, secured spaces ("design libraries") to collect, annotate, discuss, and organize private, contextual design examples and even private patterns without having to invest in custom hardware and software or having to abuse general-purpose collaboration tools.
The goal of these design libraries is similar to the purpose of the "style guide" but without the common drawbacks--monolithic documents nobody reads, hard-to-navigate and maintain wikis, SharePoint sites nobody likes or remembers, etc. Quince was designed with the need of providing design guidance specifically in mind to help designers and developers cultivate a shared understanding of their products/projects and thereby create more consistent, learnable, and just plain usable solutions currently and in the future as new capabilities and interactions are added to their solutions.
And of course, by leveraging Silverlight as a RIA platform, we also have tried to make Quince itself a pleasure to use--useful, usable, and desirable.