For MacBook protection as simple as a sleeve, you need something that has enough padding to  withstand a drop. The Thule MacBook Sleeve ($50) provides such protection and then some. This rugged sleeve is fully padded with with semi-rigid, high density molded foam to help safely transport your MacBook to any destination. Better yet, its outer shell and reverse zipper are both weather resistant, and its clamshell design allows use of your laptop while in the case.