Take a look at Carradice bags like the Barley or Pendle. That is what I use for commuting, and they have room for all of the gear I need about 99% of the time. I also keep a supply of clothes and food at my office, which I restock on days that I drive. You can mount a Barley or Pendle bag to virtually any bike, but I would recommend using one of their "Bagman" racks to support the bag and keep it from swaying. Acorn also makes a bag that supposedly doesn't need a support rack, but they are hard to buy (limited supplies) and more expensive than Carradice. You can get much better prices and selection on Carradice bags by shopping from British on-line shops such as Wiggle or SJS Cycles (or directly from Carradice).

See the photos I've attached of my Barley bag, mounted on various bikes. The Merckx and De Bernardi bikes are racing frames with no mounts for racks and fenders and typical racing geometries.