and about half have the double outlet 5 volt USB chargers plugged in, pretty much permanently. I installed half a dozen sockets initially for various uses, but the explosion in rechargeable devices with USB plug feed has made a lot more necessary

You can do a lot with 12 volt extension cords with multiple outlets, as shown in Larry's but that gets cable all over the place.

Our latest addition was where I added 5 "lighter" sockets to a shelf. We have not filled all of them yet, but we have been close.

Blue Sea's double USB outlet looks useful, but do not go to the trouble of wiring one. If you do it, install at least 3 at the same time, for little extra work (if space permits)

When buying the double USB chargers to plug into "lighter" sockets, watch the quality. Get one with 10 watt output, or some devices like iPads will charge VERY slowly. The good ones are OK, but there are a lot of trashy USB chargers around.