Manduka Large Mat Sak
I love this mat bag. It has two big zippered compartments - one for your mat and one for whatever else you bring. It's good because they're separated, so your change of clothes isn't rubbing all up on your yoga mat and vice versa. Plus, the mat compartment is vented so it can air out, and the strap has a really cushy shoulder pad on it so it won't wear into your shoulder in between the studio and home.