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Under the hood, we were able to build the foundation of a browser that runs today's complex web applications much better. By keeping each tab in an isolated "sandbox", we were able to prevent one tab from crashing another and provide improved protection from rogue sites. We improved speed and responsiveness across the board. We also built a more powerful javascript engine, V8, to power the next generation of web applications that aren't even possible in today's browsers.
A beta version of Google Chrome for Windows will be released tomorrow.
Official Google Blog