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!
Be sure to also check out the list of import tools.
Alejandro built a Prometheus exporter for Firefly III.
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.
OCR Firefly Mobile
OCR Firefly Mobile is a little tool to help you enter transactions into the mobile app automatically.
Abacus iOS App
Abacus is an iOS app created by @victorbalssa that allows you to use your Firefly III instance from your iPhone.
Lychnos is a simple web front-end for Firefly III made by @davidschlachter, built around annual budgets for categories. Supports creating and viewing transactions alongside its budgeting features.
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.
Telegram bot for Firefly III
The Telegram bot for Firefly III can create transactions through Telegram.
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.