Baking soda is a low cost product that has many uses. I like to use it for cleaning my house, in the laundry and for my hair.
How do you use baking soda?
How don't I use baking soda?
Everything people have listed so far and:
I scrub the kitchen sink with used citrus rind and baking soda (makes it shiny),
clean tub/tile/toilet/sink in bathroom with it.
Get stains off the linoleum
scrub pots and pans.
put some in my home-mixed laundry powder to help with the hard water issues
soak up pet stains (back when I had a pet) because a pile of it soaks up most of the urine and covers the smell
exfoliant for dry skin patches
tummy-soother (how-to for this is on the box)
make amusing drain-volcanoes when the sink drain is plugged by pouring in a cup of baking soda followed by a cup of hot vinegar
put a packet of it in with stored clothes to keep them fresh smelling
make a paste and put on bug stings/bites
I'm sure there are more but I can't think of them right now.