Can I copy by highlighting and paste by middle click on Mac OS X?
I wrote a free little C program that does something similar to Gilligan's answer. Whenever you drag-highlight or double-click text, it copies to the clipboard buffer. Then you can middle-mouse-click in any window to paste it. It is called "macpaste" and on Github (https://github.com/lodestone/macpaste). It works globally for every program I use that has textual data.

In iTerm2, disable their middle-click in Preferences, otherwise you'll get double pastes.