Make efficient snapshots of a directory to another location, using hardlinks to the previous backup when appropriate. Sorta like netapp .snapshots. Now works on (at least) FreeBSD, Solaris, Linux. Needs to be tested elsewhere.
This is similiar, but not, to the various rsync snapshot methods out there now. rsync benefits: someone else maintains the code. rsync downside: it scans your source and dest (entirely) before starting the job, which can be _very_ costly. rsync benefit: can operate across multiple hosts (my tool can’t).