After lightsabers, the items I found to be the coolest in the sci-fi genre were Tricorders. The Star Trek gadgets were practical, solved a heck of a lot of problems, and they looked cool. While there have been many replicas, someone has finally built a working version. It’s not a precise duplicate of what you see in the series, but the spirit is the same. This functional, Star Trek-style Tricorder was designed by Peter Jansen.
The most basic description is that the Tricorder is a self-contained portable sensor. It has GPS, and it can measure ambient temperature, humidity, air pressure, magnetic fields, surface temperatures, colors, ambient light level, ambient polarization, acceleration, direction and distance (ultrasonically).
View a photo of the Mark 2 version (a work in progress—runs on Linux) along with a video showing how it works after the break.
The Tricorder is an open source project, so you can learn how to build your own at The Tricorder Project. However, Jansen clames that a version could be mass produced somewhere down the line.