Sonic Screwdriver
"Perhaps writers for the new seasons hit a wall one day though and decided to bring it back, as they've featured the handheld tool in an array of episodes since 2005."
Call me cynical if you want, but a wand with an LED on the end and makes a high pitched warbling sound is both desirable as a toy (when backed by an entire TV series showing how cool it is) and incredibly cheap to make, which makes for a healthy profit margin.