I keep thinking up brilliant ideas.
Or at least thinking of brilliant ideas someone else should think of.
See if you can get on and invent one of these for the good of mankind......

"Find me" transponder tag
A development of the old whistling key-chain.
5 sub-ricegrain-sized RF transponders and a car alarm remote control-sized radar device with an audible read-out (low frequency=far away,higher frequency=closer) and 5 buttons (tagged Glasses, Keys, Wallet, Watch, Mobile phone or other desirable but easily-lost items) could find the missing items.
A further development could be individual codes (say, IPv6 addresses or EAN codes), to allow items to be uniquely identified.

RDF Police detector
Mountable on the car, one aerial at each end, responds to radio locator beacon on Police cars.
It simply tells you when there's a Police car near.
Very simple, 2-400 yds notice is all you need.
Just so you know they're there before they know you are.
Highly illegal, of course.....

Switchable Give Way on traffic lights
Have you ever sat at traffic lights on red and wondered why they are red when there's no traffic coming the other way?
Have you ever been tempted to just, well, go, anyway?
What a waste of time.
Most traffic-lights are rush-hour optimised and for the other 20 hours per day are a complete waste of time.
A small addition to the Highway code and the traffic-light processors would add a non-rush-hour option to all traffic-lights.
Flashing red means you must Give Way, simple as that.
So, for example, on roundabout traffc-lights you alternate between 30 seconds of green and 30 seconds of flashing red. When it's green you can go, when it's flashing red you Give Way (i.e. stop if its's busy, go if it isn't).
This has to be more efficient than turning the Northern section of the M25 in to a dual-carriageway by adding a Bus Lane that's used once every 10 minutes by a bus with 3 people on board...

Taxi FM
I don't use taxis much but I can't help thinking advertisers are missing a trick here: why not have a "back of taxi" radio station that people in the backs of taxis would have to listen to. Wall-to-wall advertising. Punters wouldn't like it but actually in most cases they wouldn't know, they'd just think it was the driver was listening to.