Writing. a revolution in communication, but also in thought. easier to think in abstract and complex terms when you can write stuff down. Applications in diplomacy, record keeping, commerce, mathematics, religion...
The Internet. I don't think we've seen just how far this thing can go. It was primarily communication at first, and then commerce became valuable... now it's changing the way we produce and tearing down old industries. I wouldn't be surprised if a global network enables a whole new military doctrine. I'm not ashamed to speculate that this will be a big one, when the dust finally settles.
Agriculture. Definitely allowed an abundance of food, but it created a huge ripple effect. You can't have a specialized economy without an abundance of food. Without agriculture, everyone would spend their day trying to secure the necessities of life. Agriculture lets a few people focus on that, while other people focus on other things -- Mining, Warfare, Science, Building, Trading...
Electricity. This changed the way people work. It allowed new methods of communication, and thus changed warfare. In terms of sheer impact, this was a big one.
Printing Press. Strictly a communication technology, even more narrow in its use than writing and the internet. But still just as revolutionary. Allowed the scientific revolution, the rise of democracy, the criticism of religion, and even had some interesting cultural and artistic impact.
All pretty non-controversial candidates for top 5, if you ask me. But fire, vaccination, money, paper, double-entry book keeping, religion, statistics, factories, evolution, and contraceptives are all pretty important inventions/discoveries.
