Are there any up-to-date rsync frontends or similar for OSX?
I think Carbon Copy Cloner 4 would do the trick here.
Its major selling point is that it creates bootable backups, but it can copy folders with data also.
If I understand right, the equivalent of that Arrsync merge function would be CCC's &quot;SafetyNet&quot; feature, with &quot;Prune archives in the SafetyNet when:&quot; set to Never.
(Edited:) I think the closest equivalent of that Arrsync &quot;unidirectional merge&quot; function would be:

use Advanced Settings
set &quot;SafetyNet&quot; to &quot;Don't delete anything&quot;
set &quot;SafetyNet pruning&quot; to Never.

From http://bombich.com/kb/ccc4/release-notes

Carbon Copy Cloner 4.0.2
... When the &quot;Don't delete anything&quot; Advanced SafetyNet option is specified, older versions of modified items are not deleted, they are moved to the SafetyNet. SafetyNet pruning is now available for tasks configured with this option.

So if I understand right (this will need testing!), this will add new files to the backup, put new versions of changed files into place, and move the out-dated versions to the SafetyNet folder, where you can delete them.  I think this achieves an &quot;ever-growing collection of all my past projects&quot;.
CCC calls rsync to do the actual work, so it is fast.  You might consider it &quot;standalone&quot;, though, as it doesn't give access to all rsync options.
I think the UI in the new version 4 is great.
There is a 30-day trial.