Open Banking API
Moov-io Accounts API
There is a whole family of plain text accounting ledgers inspired by Ledger-Cli. They can be found here https://plaintextaccounting.org/
Some I find particularly interesting
- Ledger https://www.ledger-cli.org/ which started the whole family of tools.
- Hledger https://hledger.org/
- Beancount http://furius.ca/beancount
- Abandon https://github.com/hrj/abandon
- Transity https://github.com/feramhq/transity
- Tackler https://gitlab.com/e257/accounting/tackler
Square had a blog post on their internal ledger service.