See Repositories where I am Collaborator

A simpler place (as per today) to find private repositories you are collaborating on is to go to https://github.com/ (do not include your username, but make sure you are already logged in). On the left hand side, you can find a search interface or click "show more" to see them all. It's weird that as per today, if you try to do this from the Repositories you won't find them.

Even though it won't show up in YOUR repositories, it should appear on your github dashboard (click the github logo), on the bottom right. There should be a list that has buttons at the top to filter all, public, and private repos. Try clicking the private button and it should be there.

Note that if you have opted in to the new dashboard, the above doesn't apply, but you can see the repositories (owner and collaborator) on the left..

You can see the repositories where you're a collaborator. To do this, head to the Repositories section of your account's settings (https://github.com/settings/repositories).

There should be a Leave action button aside each repository you're a collaborator on.