There are loads of riding bags available, but this seriously tough sack from Vaude is definitely one of the better ones.
For a start, the 600D polyester fabric looks as good now as it did when we started using it several months ago. There are two side pockets for bottles, a couple of internal dividers for the two main pockets and plenty of straps for body armour. There’s an mp3 player pocket too, and a bigger 24 litre version is available as well.
Either way its stable when it’s fully loaded thanks to the padded waist strap and a sternum strap. The stiff backpiece means stuff doesn’t dig into your back. It’s expensive considering you don’t get a bladder though.