I reuse them and have plastic bags filled with plastic bags, just waiting for the right opportunity.
You can use them:
* around the house (line small garbage cans, hold muffins, organize rags),
* when running errands (bring them with you when shopping, as a make-shift rain bonnet, to hold your wet umbrella),
* outdoors (kneel on them to protect your pants, cover your car's side mirrors so that you don't have to scrape ice off in the morning),
* for pets (pick up after your dog, to throw away used kitty litter),
* for donations (library, food pantry, thrift store: they can all use extra plastic bags!)
* when traveling (to protect shoes, separate your dirty laundry from clean clothes).
For more ideas, I recently wrote a hub about 50 ways to reuse plastic bags: http://hubpages.com/money/50-Uses-For-Plastic-Bags