Sign in with Facebook or Google: Plenty of sites and apps offer this login convenience, and you can connect all kinds of apps to these and other social accounts including Twitter, Dropbox, LinkedIn, Instagram, and more. Connecting all your apps has its benefits but chances are you've got ones you're no longer using and should remove, simply to protect your personal info.
When you use Google or Facebook or another service to log into another app or website, you have to give it some permissions to your basic personal information stored on those services. Depending on the security setup, sometimes connected apps can be a liability (as was the case with unauthorized third-party apps causing a security breach on Snapchat).
As someone who regularly tries out new apps and services, I've got more apps connected to these major services than I even remember installing. Some of them don't even exist anymore, like Everpix, and yet it's still apparently able to access to my emails, photos, and personal information.
As David Strom wrote on IDG Answers, you can head to the applications and permissions screens for Facebook, Google, Twitter, LinkedIn, and Dropbox to do some app cleanup. Uncheck the ones you're no longer using or perhaps even don't remember what they're for.
An easier solution might be to use MyPermissions.org. The site shows you which apps you've connected to 11 major services and provides the links to remove apps and clean up your permissions. You can get alerts when apps access your private info and also audit your apps permissions on the go via the service's mobile apps. MyPermissions also breaks down the types of permissions apps and sites are accessing, such as knowing your location or accessing your files.
For those concerned, MyPermissions doesn't have access to your site passwords or logins; rather, it uses your existing sessions (you're already signed into) to revoke or edit the connected apps.
I've got 91 apps connected on Facebook (!). Be right back.