I have used all my activations. Which sites are they?

The plugin's Licence screen lists them. Every site using your key is there with the date it was added and when it last checked in, and each one has a Free button beside it. "Last seen" is the useful column when you are deciding: a site that stopped checking in months ago is the one to let go.

Freeing a site takes it off the licence there and then, and the freed slot can be used straight away.

The list also appears when an activation is refused. Type your key, press Activate licence, and if the licence is full you are shown the sites using it — with the same buttons — instead of being told to go and find them in your account.

Local and staging copies are listed too, marked as free. They never count towards your limit.