Improving GUIs In Open Source?

frekio asks: "I'm a 4th year undergrad at UCSD and am taking part of an ambitious cognitive science course that moves us, as groups, to redesign an interface with cognitive processes, usability and efficiency in mind. My idea was to contribute to Open Source with this project by assisting in a GUI design/redesign. We need to work with something that has an active user community which could provide feedback about changes we made or propose, and therefore must have a decently large userbase. What are some projects out there that provide software which people use day-to-day and could use more time and effort put into the User Interface?"