My bet crate (for "bynary expression tree") was noticed by nobody, maybe because it's a little abstract, but it could probably get more use.
It allowed me to implement query languages in a few dozens lines in several applications, for example queries like this one in lfs:
lfs -f 'size>2TB'
or
lfs -f '(disk=ssd | type=xfs) & free < 2.5G'