If you use Google Drive to collaborate as often as we do around here, you probably don't have time to go back and clean up permissions or old files when you're through with them. WhoHasAccess is a web app that shows you all of the people and permissions you've granted so you can tighten up your security.
WhoHasAccess is a side project of the folks behind previously mentioned Hojoki, and it's a pretty simple tool. Just click the big "Scan my Google Drive now" button, grant the web app permissions to your Drive files and folders, and sit back while the tool does its work. As soon as the scan is completed they delete the data they collect and remove their access permissions (you can do the same on your account permissions page). The scan may take a while depending on how much you have stored -- but they will email you when the results are ready.
When the scan is completed, you'll see an image much like the one above, where you can look and see who has permissions to your files and the addresses you've granted access to. Click on any individual to see a specific list of files they have rights to on the right side of the page. You can even revoke all of their access rights with a single click.
WhoHasAccess is a simple tool, but it's fast and effective. If you don't necessarily like the idea of old coworkers having lingering permissions to your work, it's a huge help.