Might as well share the list with y'all, make it meta.
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2Checkout | https://www.2checkout.com
Adyen | https://www.adyen.com
Amazon Pay | https://pay.amazon.com
Authorize.net | https://www.authorize.net
Balance | https://www.getbalance.com
Braintree | https://www.braintreepayments.com
ChargeBee | https://www.chargebee.com
Chargify | https://www.chargify.com
Checkout.com | https://www.checkout.com
DigitalRiver | https://www.digitalriver.com
Dwolla | https://www.dwolla.com
FastSpring | https://fastspring.com
GoCardless | https://gocardless.com
MangoPay | https://www.mangopay.com
Mollie | https://www.mollie.com
Opayo (previously SagePay) | https://www.opayo.co.uk
Paddle | https://www.paddle.com
Payoneer | https://www.payoneer.com
PayPal | https://www.paypal.com
Qonto | https://qonto.com/en
Spreedly | https://www.spreedly.com
Square | https://squareup.com
Verifone | https://www.verifone.com/en/us
White | https://whitepayments.com
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Observations from my data collection:
Sassy appears to have been acquired by FastSpring. Balanced Payments apparently went under but I found another payment service called "Balance" instead. Paymill is defunct but their Github org has a bunch of code. No idea what happened to Spryng Payments but Spryng still exists.
It's also worth mentioning that a true "Stripe alternative" simply does not exist. No one does everything they do and I'm pretty sure most services are lacking in the developer documentation department. Still, having options is always great.