You can leave a project that you’re a member of by selecting the "Leave Project" option under the options menu, which is revealed when you hover over the project's thumbnail on the Projects page.
Zeplin's Summer Breeze release has a new options button near the project name where you can manage your project and take actions, including leaving it. Click on the settings menu near the project's name and select "Leave project."
☝️ If you're on the Free plan or deprecated Starter and Growing Business plans, once you transfer your project to another user, you'll stay as a member of that project with an Admin role. You can still leave this project if you don't want to see it under your workspace.