I’m not much of a mouse user, but a colleague using emacs for OS X wanted to replicate the normal X11 behaviour that linux users are familiar with – text selected with the mouse is automatically copied to the system clipboard. It turns out to be as easy as adding one line to your emacs config file:

(setq mouse-drag-copy-region t)

From the help for the variable (C-h v mouse-drag-copy-region)

If non-nil, copy to kill-ring upon mouse adjustments of the region.

In other words, highlight a block of text and it is copied to the kill-ring (Emacs’ internal clipboard). This also copies to the system clipboard on the Mac so you can then paste to other apps.