Which GUI library does Google Chrome use?

On Linux it uses GTK+, on Mac OS X Cocoa, and on Windows a custom views library, see http://www.youtube.com/watch?v=WsvNebq1dRg and also documents from http://dev.chromium.org:

• http://dev.chromium.org/developers/design-documents/chromeviews
• http://dev.chromium.org/developers/design-documents/views-windowing

On Windows WTL for the (well...) chrome, and WebKit for the HTML rendering.

WTL on Windows, as Shay said. All UI in the Mac and Linux versions is being written from scratch in Cocoa and GTK+ respectively. Only the logic (and WebKit, which renders the web pages) is fully cross-platform.