I wanted to put this in a separate thread so it's easy to find in the future. Credit to @LUXMAN for asking about the adapter.

Firstly, I LOVE Plugshare. Tesla has a really nice supercharger infrastructure but especially with the Model 3 not getting free Supercharging, it's nice to see other places to get level 2 charging (for free a lot of the time). I wanted to see how the J1772 adapter works so I looked up my location on Plugshare and found a bunch of local business that have Tesla chargers installed and will let you plug in for free. These guys aren't listed on the Tesla map and are really useful for certain areas that mau not have a big Tesla SC presence like for @RiggerJon in Austin.

So the J1772 adapter is included with the Universal Mobile Charger that comes with the Model 3. Its small and easy to lose and costs almost $100 to replace so DONT LOSE IT!!! I leave my UMC plugged in at home. It has huge metal blades to go into the NEMA 14-50 receptacle and frequent insertion/removal will cause premature wear. I primarily charge at home and when I charge on a road trip I use Supercharger or destination chargers so I don't need my UMC. I leave the J1772 adapter in my glove box in case I want to use a non-Tesla charger. It I haven't used it yet until today. The plug looks like this:
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Using Plugshare I found a local Nissan dealer with a J1772 charging station. Many BMW service centers also have J1772 chargers.
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The black switch on the charger handle allows you to remove the plug from the station. The adapter and the charger plug:
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There is no button on the charging handle like the Tesla handles so you need to manually open the charge port door from the screen or app (or if like the S/X, press the charge port door when the car is unlocked - not sure if the 3 has this feature). Plug it in just like the Tesla charger and the adapter latches locked and the green light comes on. Success!
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Being that this charge station was supplied by an active Nissan service station, voltage was just around 200 volts. Presumably charge rate would be higher at a full 240v. J1772 preset to max of 30amps. I reached 16 MPH charge rate with A/C off at this lower voltage.
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No button on the handle to stop charging so I manually stopped charging on the screen. Light indicator now Blue to indicate not charging but how the heck do I unplug it without the button on the handle like a Tesla charger?
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I pressed the black button on the charging handle to unlatch it from the adapter and detached the handle from the adapter. Voila! Adapter stays in the charge port and light turns white and you can hear the lock disengage. Pull the adapter out and drive! For @LUXMAN, if the car is locked, the adapter stays locked to the car even if the J1772 handle is disconnected. Nobody can steal the adapter.
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How do the Nissan people feel about you using their charger?

 

If you look at PlugShare and the history of who has used a particular charge station at a car dealership, most are not the same brand as that dealer sells.
These are also probably the charge stations most likely to be blocked by other cars (inventory cars) and likely locked behind a gate after hours in many places, so probably best not to count on them being available.

 

@Brokedoc, I found out that if you press on the charge port door while the car is unlocked, the port will open. Also you can hold the trunk button on the remote a few seconds and that will also open the charge port door.

To stop charging, you can briefly press the black button on the J1772 handle without pulling on it until the color of the port turns light blue. Then release the black button which latches onto the adapter and pull.

 

I made a gif a while back to demonstrate this:

 

Is it possible to help other people charge with this adapter?

For example, my workplace has several L2 chargers in its parking garage. Currently I charge my Leaf there, but I will also charge my Model 3 there whenever Tesla finally gets around to making enough

To make sure I can charge, I get to work early and plug in. The spot next to me is designated as “EV overflow parking” and is designed for EVs to park in so they can charge after the person next to them finishes. My Leaf, having a small battery, is normally done charging by lunch so I go down to unplug it. Usually the person in the EV overflow spot has left their charge port open so I can plug them in after I unplug my car. Yesterday for the first time I encountered a Tesla (Model S) waiting to be charged but I couldn’t get it to charge. The owner had left their charge port open and their J1772 adapter in the charge port. Unfortunately when I plugged the L2 charger into the adapter their car did not start charging. I was pretty sure unplugging the adapter and plugging the whole contraption back in would have started the charge but I couldn’t do that as the adapter was locked in place. Is there something I can do to get charging started when someone has left their Tesla ready to be charged like this? Or does the owner have to come along and start the charging process?

I don’t imagine people will want to leave their cars unlocked just so some stranger has the ability to give them a charge so I hope there’s a way to make this work when the adapter is locked in place…
Charging someone else’s Leaf or Bolt is easy if they want you to (if they leave their charge port open). Charging someone else’s Tesla should be easy too! (I’m sure there are no issues at superchargers or destination chargers but don’t think most workplaces have either of those…)