Easy to use free and open source web based double entry accounting software written in PHP. Seems simpler than, say, Firefly-iii. Can generate reports on demand.
A REST API for the double-entry bookkeeping software beancount.
A double-entry bookkeeping computer language that lets you define financial transaction records in a text file, read them in memory, generate a variety of reports from them, and provides a web interface. In theory, if it can read a text file, it can manipulate your accounting "database." More of a standard or file format than an application because you can do it all in a text editor if you really want.
The reference implementation is written in Python and seems to have a webapp.
Fava is a web interface for the double-entry bookkeeping software Beancount with a focus on features and usability. Generates expense reports, graphs, tracks your money, assets and accounts, even lets you run queries. Has an API but it's for internal use only
Personal finance software written in Java. Thus, it'll run pretty much anywhere. It comes as a single .jar file, so you can drop it into an encrypted volume along with the data files, which I suggest that everyone do. These ARE your finances, after all.
A movement and meta project for double entry accounting using plain text. cli Aims to be simple and as easy to do as possible. links to many implementations of this data format. Exportable and importable. faq wiki
foss software cli implementation of Plain Text accounting. Tries to be as simple as possible to use. A reference implementation, if you like.