At the top of the page is a search bar with an option to search your Workspaces, the Workspaces for the class you are currently viewing, or Publicly Shared Workspaces. This will search both Workspaces, cards and groups for the term you specify. To find all public Workspaces, leave the search bar empty, choose "Public Workspaces" and hit enter. Or, just click here.
Then select the "Public" tab.
You can filter the search results to show Workspaces, Cards and Groups, or any combination.