These gadgets changed the world! In 2011, Popular Mechanics gathered a panel of experts—esteemed inventors, designers, tech gurus, even a moonwalking astronaut—to create a definitive list of 101 gadgets that changed the course of history. The feature was so popular that it became a History Channel TV special and a PM iPad app. Now, the magazine's editors have finally expanded and updated that list, adding 49 new entries. Ranging from the microscope, bicycle, and transistor radio to the hearing aid, electric guitar, and Roomba, this compilation is sure to fuel debates—especially when it comes to the 10 top choices. (Spoiler alert: The smartphone clocks in at #1!)