handlebar bag or what?

[KnottedYet]

Ortlieb makes some nice ones (in a couple different sizes) that are waterproof and can also be carried like a shoulder bag. They are rather expensive, but there are similar designs by Seattle Sports and Novarra too.

If you don't need a waterproof bag, there are a gazillion to choose from! Styles from the "classic" Rivendell bags to the sleek and modern Topeak bags. (and they are generally cheaper than the waterproof ones.)