I see many here already said my answer but, I will put my 2 cents in anyway. Many times I have sewn pieces together to make a larger piece and some of those quilts are still together many years later.

Depending on size, either make a quilt using quilt as you go (I have a no-hand sew demo with pictures that I can PM you) or, what I also do with the small pieces is cut up orphan blocks or paper piece post cards. I make them 4x6, and experiment with surface design techniques on them. I take the fabric and batting behind it and do thread sketching. Very fun and a small space to experiment. Here are 2 samples. The shell is mine. The other is from Ansley Mackay.

I have also large used large zig zag to piece larger pieces together, I have been making a few purses so it is great for that and also to test for fmq. Nothing goes to waste!

I piece mine with a zig zag, the smaller pieces come in handy for me to make hot pads or pot holders, this not only uses your leftover batting but your left over fabric scraps.

I've read about dyeing or painting batting to use in an art quilt.

Yesterday I put small squares in the bottom of flower pots to keep dirt from falling through the holes and still lets the water drain out.

I use the cheapest iron on pellon I can find - on sale at Joann's for $.50, cut in strips. Cut the pieces of batting straight along the edges and iron on the strips. Actually the strips I use are leftovers from t-shirt quilts.

i save mine for table runners or smaller projects like wall ha hangings or placemats. i even intend to make a cotton rag quilt sometime when i get caught up. so i have a plastic tub i keep them in til i need them.
i guess my moms waste not want not attitude lives in me )))
carla

I just put the leftover scraps in a piece of fabric that is approximately the size of a pillowcase and fill it almost 3/4 full and then close it and give it to the homeless animals as a bed.

darlene

This is a timely question for me. I was just wondering the other day what was I going to do with the left over batting from the baby quilts because my quilt size is never the same size as the batting is. Thanks for asking it.

I cut all my leftover batting into 6" squares with my GO cutter. I love to make baby rag quilts, and even some larger ones, and when I have enough squares cut, I cut my flannel 8 1'2" rag pieces with my GO, and then build a quilt. I love my GO cutter.