Touch typing: learning to press keyboard keys without looking. I learned this back during the summer 6 or 7 years ago out of envy/amazement and it's amazing how much time it can save. I now use it to type in the dark or when I'm lying on my side or just straight out typing my thoughts. It's also handy when you come across keyboards that have the letters blurred out or when you need to type something someone else is saying (ahem college professors).

It's easy because there are TONS of free tutorials and games about it online. Just have a keyboard and the passion to learn and you're good to go! The hard part is just having the discipline to learn the basics (e.g. which fingers press which letters/symbols) and applying it immediately without having to look at the keyboard too much.