The Sonic Screwdriver is the nigh ubiquitous multi-tool of the indefatigable and polysyllabic Doctor, of the long lived BBC program Doctor Who. First appearing in the program in the "Fury of the Deep" episode, this small device has been used to pick locks, repair complex machines, electronic jiggery-pokery and even unscrew things. The original device is a metal cylinder with some odd buttons and at the top there is a round microphone bit, or perhaps an ear examiner. In the new series the sonic screwdriver has been made a bit sleeker, replacing the old cylinder is an even shinier one, with light that glows a nice technological blue.
The device was invented by Victor Pemberton for the above mentioned episode of Doctor Who and he has sadly not been widely credited for the invention as he has vocally complained. In 1982 the sonic screwdriver was written out of Doctor Who by producer John Nathan-Turner in the serial episode "The Visitation." Nathan-Turner removed it from the show because he was afraid that it lent the Doctor with too easy an out in too many plots. Land Mines, Daleks, imprisonment, the imminent destruction of the ship, planet and/or universe; the sonic screwdriver was indispensable in dealing with almost all of the Doctor's problems. And so the sonic screwdriver was confined to the props box until the original show's end in 1989.
It was brought back, slightly re-imagined (it telescoped for one thing), in the 1996 American TV movie of Doctor Who. And it survives in its current incarnation on the current program. Its functionality was also heavily expanded, it can now extort money from an ATM, act as a medical scanner, interface with computers and remotely operate programs on the TARDIS.
Whether or not it is too convenient a plot device is a matter of opinion. But in the opinion of this writer the sonic screwdriver, while a tool for everything, is simply a device for the Doctor to use to interface with the technologies across all of time. While this seems to be boundless utility, it doesn't solve the Doctor's problems. The Doctor needs it to solve problems in many cases, but it is he who solves the problems, not the funny little vibrator he carries with him.