bKappa
Cross-platform incremental backup
... and PowerShell. While it saw real use, bKappa isn't tested enough to be your main backup software. If you are looking for that, BorgBackup and restic are good options as of 2024.
Warning! Old 1.x versions of bKappa are insecure. Do not use them. Because backup metadata is serialized with the "pickle" Python module (https://docs.python.org/2.7/library/pickle.html), a malicious backup can take over your machine.