Google Tag Manager is a tag management system (TMS) that allows you to quickly and easily update measurement codes and related code fragments collectively known as tags on your website or mobile app.
Modernizr is a JavaScript library that detects the features available in a user's browser.
reCAPTCHA is a free service from Google that helps protect websites from spam and abuse.
Twitter is a 'microblogging' system that allows you to send and receive short posts called tweets.