To be so slim, RadTech's RadSleevz cases for Apple laptops do a good job of protecting the exterior finish of your MacBook, MacBook Air, and MacBook Pro from bumps and dings.
They're made of an ultra-micro fiber called Optex, which was specifically designed for polishing plastic optics without scratching. A RadSleevz slips onto your laptop like a glove and protects its outsides from a larger case's zippers when you, for instance, stow it for traveling (plus, it's TSA Checkpoint Friendly). Available in six colors for $24.99 and up (depending on your laptop model), it can also be used to insulate your lap or furniture from hot notebook bottoms.
The RadTech case is extremely lightweight (about 36 grams), breathable, fairly flexible (though it's a bit stiffer than I'd prefer), and rugged enough to offer some protection for your laptop wihtout adding a lot of bulk. The RadSleevz is svelte enough to be folded and stashed in a pack, purse or pocket when it's not being used. Plus, you can wash it hundreds of times and it's still good to go.
You shouldn't go running across campus or through the airport with your MacBook, MacBook Air, or MacBook Pro ONLY in the RadSleevz because, after all, it is a sleeve, not a full-fledged case. However, it does make for a handy tote when you're moving your laptop around the office or house.