There are a lot of envelope-style sleeves for your Mac: leather from Bally, a sturdy vinyl version and plenty of handmade fabric ones, too.
A nice variant on the theme: these from Spanish company iSobre have a spark of color on the inside. Hand-made leather, they’ll provide protection for the MacBook, MacBook Air and MacBook Pro. They come in basic manilla, plus black or white with pink, green, or blue insides.
So you can show your the world that, yes, your computer is thinner than the presentation your boss insisted on printing out for clients.
Available online at iSobre.