I think the most useful developments are the mobile phone and the laptop computer, because with this two tools you can make a lot of actions thanks to the functions that each one has. for example:
Mobile phone:
1. Obviously use it for doing calls
2. Use it as a cd player (mp3 player)
3. Alarm clock
4. Games
5. some models as a tv
Laptop computer:
1. Sendind and reciving information
2. Do work, have fun, entertain yourself, chat, music,
this two tools are the basics for any people who works.