Sonic Screwdriver used by The Doctor
This is the tool or weapon of choice of The Doctor. It is used in many ways to do many things, such as unlocking doors, changing software, etc. The uses of the sonic screw driver are wide, being used to scan for lifeforms in some episodes such as Silence In The Library, where it was used to scan the shadows for life. The sonic screw driver is seen in one other persons hands (Proffessor River Song), and they come from the future and knew the doctor from there. Their sonic screw driver looks different to the current doctors, as it was modified to have a higher power threshold.
This device is a piece of Gallifrayian technology, and as the only surviving Time Lord of the Last Great Time War is now one of a kind. It can be used to open almost anything the Doctor chooses depending on what setting the Doctor puts it on. In several episodes the Doctor also uses it to scan various alien objects to either deactivate them or learn as much as he can from them. This Sonic Screwdriver is more or less impossible to work out unless in the hands of a fellow Time-Lord.
There are different variations in these, The Master has his own version, more of a weapon than the Doctors, The Laser Screwdriver, this can fire Laser Beams at The Masters foes, It can also age people, as seen in Last of the Time Lords and Sound of The Drums.