Mountain Lion has improved the Address Book interface, and renamed it "Contacts". It's great that you can now see groups, group members, and contact details in one window; they've done away with the awkward switching between groups. But they've retained the somewhat annoying attempt to look like a real, leather bound, address book.
Similarly, there's a better appearance to the renamed Calendars, but they've retained the silly leather bound look, and torn off paper look.
So, I've created silver skins for both of these. And I've gone one better by creating a proper installer package. You can choose which app to fix using the Customise button, but by default it'll just fix both of them.
The installer should only work on Mountain Lion, and it should refuse to change apps on other versions of OSX. Also, it requires that Contacts.app and Calendar.app are in the usual locations.
update 21 August: fixed link. Sorry!