The best screw-in light socket adapters

If you want dimming on a lamp or fixture with no dimmer, the Westek 6009B socket dimmer is the pick. If you want power where there is only a light, the DiCUNO UL Listed socket-to-outlet adapter is the one to buy, because it is UL Listed and most of this category is not. Both screw in, neither needs a tool, and both come out with you.

The whole category exists for one reason: a bulb socket is a powered connector sitting in a ceiling, a closet and a garage in nearly every home, and a tenant is allowed to unscrew a bulb. Everything here uses that.

This is a category of inexpensive parts from makers who publish very little, so the criteria are mostly about separating what is specified from what is assumed.

1. A safety listing, where one exists. These carry mains current in a fitting that is often left on. UL Listed is stated on two of the four below and missing on the rest, and we say which is which rather than evening it out.
2. Whether the light survives. An adapter that takes the socket and gives nothing back has a much narrower use than one that passes the bulb through.
3. Honest about height. Every adapter adds length between socket and bulb. That is the reason most of them fail to fit, and none of the listings mention it.
4. Published function, not implied function. A motion socket with no published detection range is sold on an assumption.

We did not bench-test these. Every figure here is the retailer listing's own, with the page it came from in the sources below, and where a maker publishes nothing we say so.

Westek 6009B socket dimmer - the pick. A full-range rotary dimmer that screws between socket and bulb, rated 100W on the listing. It is the answer to a lamp with a plain on/off switch and no outlet-level control, and to a rented fixture you are not allowed to rewire.
Who it is for: a renter with a hardwired fixture or a lamp whose socket switch is all it has. Who should skip it: anyone whose bulb is not dimmable - this will buzz and shorten its life - anyone with an enclosed fitting that will not take the extra height, and anyone who would rather reach a control than fish under a shade for one.
Cons: the 100W rating is an incandescent-era figure; the control sits under the shade where it is awkward to reach; it raises the bulb, which can stop a harp or shade sitting as before; and stock on this listing has been thin.

DiCUNO E26 to 2 polarized outlet adapter - for power where there is none. UL Listed, two polarized outlets, two per pack. The standard fix for a garage, a basement or a closet that has a light fitting and no receptacle anywhere useful.
Who it is for: anyone who needs to plug something in near a ceiling or closet light - a strip light, a charger, a fan. Who should skip it: anyone powering something that needs a ground, because these are two-prong with no earth, and anyone who still wants the light, since this version takes the socket.
Cons: no ground; you lose the bulb; a ceiling socket is an awkward place to plug into; and whatever you plug in hangs from the fitting.

DiCUNO 2-in-1 socket splitter - for a fitting that is simply too dim. One socket in, two out. The only way to genuinely increase output from a single-socket ceiling fitting without replacing the fitting, which in a rental is often the only option.
Who it is for: anyone with one weak bulb lighting a whole room and no authority to change the fixture. Who should skip it: anyone whose fitting has a shade they want to keep - two bulbs are wider and lower, and the shade usually will not go back on.
Cons: no UL listing stated; the fixture wattage limit applies to the pair; two bulbs in a confined fitting trap more heat; and it looks like what it is.

LPHUMEX motion sensor sockets, 3-pack - for a light nobody remembers to switch off. Motion plus dusk-to-dawn switching added to an existing fitting, three to a pack - enough for a garage, a basement stair and a porch.
Who it is for: anyone with an exposed utility fitting in a space they walk through with their hands full. Who should skip it: anyone fitting it inside a shade, where the sensor sees nothing, and anyone on a dimmer circuit.
Cons: no detection range, hold time or lux threshold published; adds height, so enclosed fittings often will not close; needs line of sight; not dimmable.

If the lamp plugs into a wall, a plug-in dimmer is better than a socket dimmer in every way - the control is at hand height rather than under the shade. See the best plug-in dimmers.

If you want the switch on the cord rather than at either end, cord, foot and inline switches is the simpler route.

If what you actually want is control from the wall without rewiring, that is a battery remote rather than an adapter - wireless wall remotes covers it, and renter-friendly light switch controls puts the whole set of options side by side.

If the ceiling has no fitting at all, an adapter has nothing to screw into - ceiling lights that need no wiring is the page for that.