1) Blocking numbers. Someone bothering you? Set their number on block and they'll just go straight to answerphone. So simple I bet its been done already.
2) Mobile phones for your keys. And your remote control. Remember that terrible whistling keyring thing? Well that was shite, but nowadays you could attach a tiny mobile phone to your keyring and when you lose them ring it. Thats all. Thats a fucking great idea. Obviously I've copywritten it already but get in touch if you want to give me loads of money to make it.