Just fabric you haven't cut small enough yet!
I found Bonnie Hunter's method of organizing scraps,
then I started this project two weeks ago.
I had a Huge basket of scraps.
It took me about 20 hours to organize it into this:
She recommends cutting strips in 1.5", 2", 2.5", 3" and 3.5" widths.
I cut the largest width whenever possible,
unless it made a leftover just shy of 1.5",
in which case I cut a 3" and a 1.5" strip instead.
I did not cut up yardage, just scraps.
Some largish scraps, but mostly not too big.
What I had most of was leftover backing fabric -
many very long strips. I cut lots of 3.5" strips.
I didn't have any set length for the strip, but
very few are selvage to selvage.
Bonnie also recommends making 1.5", 2", 2.5", 3" and 3.5" blocks when there isn't enough length for a strip.
I always cut the largest block possible;
for some reason, 1.5", 2" and 3" blocks were very common, and
2.5" and 3.5" were not.
And yes, I do have plans for making quilts from all this!
For me, there is nothing like doing a boring, repetitive task to get the creative juices really flowing;
plus, now I can easily see what I have.