Google Chrome 12

Two new chrome.extension methods—isAllowedFileSchemeAccess() and isAllowedIncognitoAccess()—let you determine whether your extension has increased access, which the user specifies using the extensions management page (chrome://extensions).
The chrome.windows.create() method can now take a focused value. Previously, all new windows had the keyboard focus; now you can create windows without interrupting the user's typing.
If the manifest specifies "experimental" permission, your extension can specify "panel" as the value of the type field in the chrome.windows.create() method or the Window type.
The onChanged event of chrome.cookies now has a cause parameter.
The chrome.contextMenus create() and update() methods now let you specify a context value of "frame".

Google Chrome 11

For security reasons, you can no longer call chrome.tab.captureVisibleTab() on just any tab. Instead, you now must have host permission for the URL displayed by that tab. To get the previous behavior, specify <all_urls> for the host permission.

The management API's ExtensionInfo object now has a homepageUrl property.
The management API now lets you get the icons of disabled apps and extensions. Also, you can now modify the regular icon's URL to get its disabled equivalent. See IconInfo for details.
The cookies API set() and remove() methods can now take callbacks.

The new background permission extends the life of Chrome, allowing your extension or app to run even when Chrome has no windows open.

The chrome.windows.create() method now has a tabId field. You can use it to move a tab or panel into a new window.
Note: This change was incorrectly attributed to Chrome 9 in previous release notes.

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 can now take a list of URLs, letting you create multiple tabs in the new window.
The new chrome.management.get() method lets you get information about the specified extension or app.

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.