Firefly III

Firefly III is a sophisticated open-source personal finance system designed for self-hosting on your own server. It includes a REST API for exporting data, integrating with other tools, and building custom reports without lock-in.

It appeals to technical users who want full control and aren't afraid of server setup. The REST API makes it powerful for accountants or anyone building custom workflows, but it's overkill if you just want simple budgeting.