Helmet liner for biking in the rain = substitute a plastic grocery bag.
Boot liners for very wet snowy days = substitute a plastic grocery bag or a bread bag.
Clips to keep loose pant legs out of the bike chain = substitute a fat rubber band that came off of a bunch of broccoli or asparagus.
Lunch containers -- re-used bread bags, re-used margarine or sour cream tubs, etc.
Wax paper -- re-used liners from cereal and cracker boxes.
Paper towel for draining greasy bacon -- use junk mail instead.
Hand soap -- use a tiny drip of dish soap.
Tarps for camping or for protection when painting the house -- old mildewy shower curtains.
"Extender" for pants that are just a little too small -- rubber band through the button hole and then around the button. When you have to do this, it's time to start watching what you're eating.
Almost everybody seems to have a bunch of cloth tote bags (from the credit union, from the grocery store, from the library...), which can be used in a pinch instead of a backpack or a purse.
I'm sure I'll think of more! Looking forward to reading what others have done.