The remarkable array of cordless power tools
Right around the time that I got out of high school, Makita cordless drills were taking over the world of drills. My friend Ken got a couple for his job installing car stereos, and my friend Marc and I got them too... they were 9.6 volts, and everyone had one.
Ken also bought a Makita right-angle drill, which was good for tight places, and used the same 9.6 volt battery.
Other tool companies joined in to break Makita's stranglehold on the market. Dewalt came in, Cable Porter, Craftsman, Ryobi, Milwaukee and many others. The battery shape morphed from a stick into something like a shoe, and the voltage of the tools slowly increased, from 9.6 volts, to 12 volts, 14.4 volts, 18 and 20 volts, 24 volts, 28 volts and 36 volts.
In retrospect, it would have been pretty awesome if they could have settled on an industry standard.
The increasing voltage arms race was called off when several manufacturers began using Lithium Ion (Li-Ion) batteries instead of traditional nickel-cadmium (NiCd) batteries. The Li-ion batteries are lighter, charge in less time and hold a charge for a year or more.
It has been fun to watch the "Professional" marketing change from big, heavy NiCads to lighter, quicker Li-Ions. Its like Ford introducing a tiny electric truck for the "professional" working man.
In 2010, the 18 volt Li-ion battery has become the heavy-duty standard drill battery. I've never had a great use for any other cordless tools, but the tool companies make a lot of them. Milwaukee has its M18 line, Ryobi has One+, Black & Decker has Firestorm, Makita has LXT, Dewalt has XRP
I think they recognise the thrill of swapping batteries like clips of ammunition. After the drill came the flashlight and the floodlight. Now there are a lot more:
Drill

Flashlight
Oh man. If you want to see a designer's rogue's gallery, take a look at the flashlights that tool companies make. Apparently flashlights which use drill batteries have to be shaped like a drill.

Work light

6 1/2 " Circular saw

Boom box/ MP3 player

Dustbuster-style Vacuum

Sander

Jigsaw

Rotary saw

Angle grinder

Sawzall-style Reciprocating saw

Cut off tool

Paint sprayer

Impact driver

Fan

Headphones

Stapler

Rotary Laser

Infrared thermometer

Inspection camera

finish nailer

chainsaw

Pole Saw

lopper

hedge trimmer

impact wrench

portable band saw

tire inflator

Weed Eater-style string trimmer

Portable wet/dry vac

Wet/dry vac

Leafblower

Auto hammer ryobi
Vibrator

As battery power approaches the price and power density of gasoline, I imagine the realm of cordless tools will continue to grow.
That's the state of the world for at the end of 2010, what's next?