How to merge (and not replace) folders when copying on the Mac?

Once in Terminal, type: ditto from_folder destination_folder

Ex:

ditto ~/Desktop/SomeFolder ~/Desktop/SomeOtherFolder

Infallible.

Holding down the Option key on Mac OS X Lion gives you the option to 'Keep Both' which then merges the two folders.

This was one of the new features of Mac OS X 10.7 Lion.

I'm a proponent for full-blown support for merge-replace-skip in the Finder, no question. In fact, it's disheartening that Apple's simplicity-minded philosophy actually results in more mouse movement, more clicks, and more steps to accomplish what seem like simple things. And clearly people want to see this kind of change (among many others) in the Finder, especially. Snow Leopard has made some steps towards FTFF with the Cocoa-rewrite.

Thanks to this Cocoa-rewrite, bliss has arrived in the ability to extend/hack the Finder in Snow Leopard with plug-ins thanks to SIMBL, which provides means for hacking/reverse-engineering Cocoa-based apps. So I figure it's only a matter of time until someone extends Finder to support this more natively. Regardless, Apple is still to blame for the Finder flaws, and they need to own up soon and just Fix the Freakin' Finder, once and for all! (I mean, it is so central to the OS you'd think they'd have done this by now.) < /rant >

Meanwhile, you can fool Transmit into using its merge capabilities with your local files: http://andrew.hedges.name/blog/2007/01/16/merge-folders-with-transmit