Some of the best products on the market at the moment for smartphone photography.
After a long line of Nexus phones produced in collaboration with various manufacturers, Google finally released their own branded phone in late 2016, the Pixel. Worldwide availability has been limited due to the Google Assistant becoming a main feature, but the camera is reported to be excellent, receiving the highest ever rating for a phone on DxOMark.
Alternatives: HTC 10, Samsung Galaxy S7
Fujifilm Instax SP-2
Following on from the success of the SP-1 and Instax line of instant cameras, Fujifilm made some minor updates in the SP-2. As before, it uses the same film as the Instax cameras, so no ink is required, and connect directly to smartphones. The prints are small and the paper isn't cheap, but the quality is very good for this type of printer.
Alternative: Polaroid Zip
The Shoulderpod fulfills two main rolls. It allows a phone to be attached to a tripod, and it provides a more comfortable grip for holding a phone on landscape mode, including a nice wrist strap. It's excellent quality and a reasonable price.
If you want to experiment with timelapse, night photography or other long exposures, even a small tripod makes a huge difference. GorillaPod make a wide range including some designed for phones and are idea for when you don't have a flat surface to work with.
Alternative: Manfrotto PixiSmart
Jackery Titan S Powerbank
Extensive use of the camera drains a phone battery quickly, so I almost always have an additional powerbank with me and Jackery make excellent products. The Titan S includes a USB-C port so you can charge it from the same cable as for the latest phones.
Alternative: Anker Powercore