What are some of the most useful phone accessories you have ever used?

Ring holder
This kind of bracket is convenient to use, carry around, go out to eat and drink coffee at any time can be used.
The Angle can be rotated at will, it can be folded up, for people who don't know, at first glance, it seems to be what high-tech new products...
What do you think?

OTG data line: can charge A phone to B cell phone
This is also the data line that everybody usually USES, right?
Instead of using a computer, the phone can also read usb flash drives, such as playing a movie in a usb flash drive and viewing pictures.

Today, however, it's not about reading A usb flash drive, but using A mobile phone to charge your B phone!
When you think about going out, your cell phone is out of power, but there is an urgent business need to deal with, everything has a cell phone, everyone has no charge treasure, only the data line, how can this be done?
If you have an OTG line at this point, it's easy to connect to someone else's phone and charge your phone with someone else's phone.

Positive and negative plug data line: Micro usb port can also blind
When it comes to the pros and cons, you might think it's the latest Type of type-c or apple's Lightning port, but the traditional Micro usb phone can also be plugged in!
Look at the interface design of this data line.
Unlike ordinary and roid data line design, it is designed and processed at both ends, so it can be inserted blind.

I don't want to go back and forth in the dark again and again. I don't want to try again to find the right direction.
Make the phone of traditional Micro usb interface also enjoy the convenience of the right and wrong insertion!
Cellphone lens: the mobile phone instantly transformed into a professional camera
Many people have added a phone camera to their mobile phone cameras to improve their professional and playability.
Like this:

After the addition, the phone can easily take the SLR of the micro - range effect, and various fish-eye lenses and ultra wide Angle lens, make mobile phone photos become better play, more professional!
What's more, when your phone is equipped with this kind of professional lens, it will make the phone's level appear to improve a few levels instantly.
What's more, this is a very professional level of mobile phone:
The mobile phone minutes into professional cameras!
There are actually too many mobile phone accessories.
For example, the kind of magnetic car stents that can suck up the phone that falls into the toilet.
There's also a phone memory card for apple/android phones that can be expanded at the same time...