The Deuter Speedlite 20 is the lightest backpack that works well as a backpack. It's designed by a company that make serious backpack, so they know how to do it right.
I originally considered it as a biking backpack but couldn't justify paying $80 while my other smaller back pack was still functional, then I got the MBA 13. It has the simple and well thought out design that I really like. For example, in addition to a large hydration pocket (where my 13" in a Be-ez LA robe goes) in a main compartment, it has a large stash compartment for globes, light, wind breaker, whatever. The side pockets are big enough for water bottles on a hike (I am not into hydration bag except for technical mountain rides).
The most amazing thing about this bag is not just how compact and light it is for a 20L capacity, but how well it carries the load given those attributes. There is a Delrin frame that hold the shape when empty and weigh next to nothing. And yes, it will fold to fit the back pocket of your jersey.