Is there an LLM service that allows end-users to "bring their own key"?

This would be useful for products or open-source tools that want to be "AI Enabled", but not deal with the usage based pricing challenges that come along with it.

Ideally, something "OAuth-esk" where the user could connect their BOYK account approve requested "scopes/models" and set spend/token limits.

I thought maybe this was a feature of OpenRouter.ai, but it seems like BOYK in that sense isn't for the "end-user".

If not, how have others handled making your project "AI Enabled" without providing direct inference?

I guess just having the user provide an API key for whatever LLM provider you support? Is this still against the TOS of most providers?