I carry my 13-inch MacBook Air in an old Timbuk 2 bag. The kind of bag that came around before dedicated laptop messenger bags were popular. And so I needed a good sleeve to keep the Air protected. There are many sleeves out there, but I like this Acme Made one because it's padded and reinforced. And the top of the sleeve is open which makes it easy to slide the MacBook Air in or out. (Also available for the 11-inch MacBook Air.)