In no particular order …
UK Market (Open Banking)
[1] HSBC
https://developer.hsbc.com/
[2] RBS #BankOfAPIs Developer Portal
https://developer.bluebank.io/
[3] Barclays
https://developer.barclays.com/
[4] Lloyds Bank Developer Portal
https://developer.lloydsbank.com/
[5] Halifax
https://developer.halifax.co.uk/opendata-v2
European Market (PSD2)
[6] Nordea Developer Portal
https://developer.nordeaopenbanking.com/app/docs
[7] ING Developer Portal
https://developer.ing.com/openbanking/
[8] Deutsche Bank Developer Portal
https://developer.db.com/#/
US Market
[9] Wells Fargo Developer Portal
https://developer.wellsfargo.com/
[10] Citi Developer Hub
https://sandbox.developerhub.citi.com/