Copy paste. Can't live without it. I guess most developers wear out their C, X, and V keys (along with CMD / CTRL) transferring code back and forth. It's your computer's equivalent to a person's short term memory.
So here's the catch. Most operating systems provide you with a single slot for saving a bit of data. Save another thing and boom: the previous thing is gone-diddly-gone. Away is that important piece of code, that you didn't save and don't know how to find again. That's where the short term memory comparison falls short: humans are usually able to retain a few things without overwriting them.
Clipboard managers to the rescue! They are simple tools that store every bit of copied information in a series of predefined slots, thus preserving it. Short term just became long term.
I really like Paste for Mac: it is highly configurable, takes security into account (not retaining those important passwords), and syncs with any iOS device. This tool has saved my live many, many times. I recommend it to every developer!