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
Step 1: Punch the holes
-Holes are ~1/2" (at least) from top edge so the holes don't rip out the edge under weight
-Equally space the holes. I recommend ~1.5" between holes
-There are equal numbers of holes on both bags
Punch out the smallest size holes you can get away with, where you marked. Your scrap wire should be able to just fit through the holes.