Google Chrome 9

The Tab object now has a pinned property that's reflected in various chrome.tabs methods. For example, you can create a pinned tab.
The chrome.windows.create() method's first parameter can now be an array of strings, letting you create an array of tabs.
The chrome.windows.create() method now has a tabId parameter. You can use it to move a tab or panel into a new window.
The new chrome.management.get() method lets you get information about the specified extension or app.

Google Chrome 8

We took a break for Chrome 8. No API or manifest changes worth noting.

Introduced split incognito mode as the default for installable web apps (also available to extensions)
The tabs API create() and update() methods no longer require the "tabs" permission, removing one common cause of scary dialogs

The context menus API allows you to add context menus to pages or specific objects on a page
The cookies API allows you to manage the browser's cookie system
The idle API allows you to detect when the machine's idle state changes

The chrome.extension.getViews() method can now return popup views
A new WINDOW_ID_NONE constant identifies when focus shifts away from the browser
The new chrome.tabs.getCurrent() method returns the tab associated with the currently executing script

The geolocation permission gives an extension access to the user's physical location
Match patterns can now select all schemes or all URLs
Access to file:/// URLs no longer triggers the "access to your machine" security warning, but now requires user opt-in from the extensions management page