Bike panniers--commuting essential, but can be "beaucoup expensive."
For all of you on a college/unemployed budget (or just into recycling/reusing), here are easy plans for recycled panniers.
2 oversize leather bags/purses (thrift store/great aunt's closet)
2 dowels (wood, aluminum, anything stiff enough to keep the bags from hitting your wheels)
speaker wire (scrap wire etc.: check trash, recycling, re-stores etc.)
1-2 belts (cloth)
leather hole punch