Facebook, Twitter, Google, Dropbox, and all the other web services out there can be accessed by various applications and other services. Here’s a couple of links that you can use to quickly remove anything that has access.
Your Google Account
Perhaps the most important of your accounts is your Gmail or Google account. You can use the following link to see every app and service that has access to your account. Once there, you’ll see a list like the picture above, which you can quickly clean up if you so choose.
Your Facebook Account
Visit this link to access your Facebook settings page, where you’ll see a list of everything that can access your account. Click the X next to any app or site to delete it.
Windows Live (Hotmail, Messenger, etc)
This link does the same as the rest, just for Windows Live.
There’s a lot of apps that can use the Dropbox API, and you should clean up this list.
You’ll be surprised how many apps have access to your Twitter account. Time to prune that list.
Yahoo, LinkedIn, Flickr, Instagram, Foursquare, and Flickr
For all the rest, you can go to MyPermissions.org and click the icon directly from their page to get to the settings page for that app.