the invention of the virtual machine. Coders best friend. Hackers worst enemy. Governements hardest thing to track. The overall best thing to mess around with.
CD, hard drive, floppy disks (SLOW!), modem (without this Internet wouldn't exist), TV, phone, WiFi, smoke alarms, MS-DOS, Windows, Mac OS, Skype, Java (without this Minecraft wouldn't exist), PHP, Wikipedia.
Hm. Microscopes. In my opinion. How many lives have been saved from research using a microscope cannot be counted. It's incredible.