There are a lot of interesting tools and apps built around the Firefly III API. Here's a list, in no particular order.
Is your (favorite) tool not on the list? Submit a PR!
David built a tool to send you a monthly overview of your expenses by category.
Firefly Personal Finance
Firefly Personal Finance is an android app created by @mconway that connects to your Firefly III installation.
Photuris III is an android app created by @emansih that connects to your Firefly III installation.
Bots and tools
Firefly III CLI
Firefly III Command Line Interface is a tool made by @afonsoc12 that allows you to interact with Firefly III over the command line.
PHP API library
The PHP API library is a library developed by StanSoft.BG Ltd that allows you to easily authenticate to Firefly III over OAuth2.0 and use the API in your own app.
The Firefly Bot can create transactions through Telegram.
iOS Shortcut to create transactions in Firefly III.
All import related tools are listed on the page on importing data.
Other tools developed by the developer of Firefly III are listed on the other tools page.