Google has recently released a plugin for Internet Explorer that renders web pages using Google Chrome. The plugin, called Google Chrome Frame, is open-source and the code is publicly available.
By looking at the code, it's obvious that the plugin is not limited to Internet Explorer. There's also a NPAPI plugin, which should work in browsers that support NPAPI: Firefox, Safari, Opera and other browsers. Here's a comment from one of the Chrome Frame files:
"ChromeFrameNPAPI: Implementation of the NPAPI plugin, which is responsible for hosting a chrome frame, i.e. an iframe like widget which hosts the the chrome window. This object delegates to Chrome.exe (via the Chrome IPC-based automation mechanism) for the actual rendering".
Instead of asking users to download a different browser, Google and other companies that develop complex web applications can ask users to install Google Chrome Frame. This way, Google Chrome runs invisibly inside another browser.