Requested feature overview description.
The ability to have multiple API keys, all of which can be separately regenerated or deleted.

Why would it be useful?
It's safer to use a different API key with each application, that way you can delete a key when you're done using an application without any interruption to other services. If the API key is logged along with requests, it could also be used to work out which tool you've trusted with your API key has been compromised in the case of abuse.

It would also be useful if there was an option to set permissions on the keys (read only and etc) but that's probably more fitting for another feature request.

What part(s) of the site page(s) are affected?
e621 API

I vote this up. I don't need this, but I like the idea.

Sounds useful, I though about it a bit. Multiple api keys aren't difficult to implement, I think there's just an arbitrary restriction in the code somewhere for that right now (and the fact that the ui doesn't account for more than one). Readonly and readwrite also sounds good and is also easy to implement since that can basically use the logic from the readonly mode the site already has. Saving the approximate time the api key was last used and being able to give api keys a name will help with distinguishing them.
I have no idea when I will spend time on this, but I've added it to the list of things I want to do eventually.