I run a REI Flash for my 10 mile commute - http://www.rei.com/product/778466/rei-flash-18-pack

It's super light, packs down to nothing when I don't need it, and has plenty of room for my change of clothes/lunch/netbook. Also is hydration system compatible. My wife and I also use it as a daypack when hiking/backpacking.

I used to use a larger backpack (also REI) and more recently two different Timbuk2 messenger bags - the Flash beats them all in versatility and comfort. Best of all it's only $30 (and cheaper if it's on sale or you have a dividend to spend!).