Shiira is a web browser based on Web Kit and written in Cocoa. The goal of the Shiira Project is to create a browser that is better and more useful than Safari. All source code used in this software is publicly available.
One of the most impressive features of Shiira is Tab Exposé. Tab Exposé shows all open tabs the same way that Exposé shows all open windows.
Shiira 2.0 requires Mac OS X 10.4, because one of the purposes of the Shiira Project is to leverage latest Mac OS X features, ex. Core Data.
Screenshots can be viewed here.
More info and download link (upper right of page) here.