Actually, for a lot of devices, there is an emerging standard for 5V power: USB

I have a Cig. lighter plug with a USB A connector on it, that I use variously to charge my cell phone, run my GPS, &c.

Conveniently, my laptop has standby power on USB when plugged into AC power, so I can just carry one power brick for the laptop, and use the 4 convenient charger outlets on the side.

And the original iPod (at least) did the same with 12V and Firewire, though that's less common.

-Z