Apache is a free and open-source cross-platform web server software.
Google AdSense is a program run by Google through which website publishers serve advertisements that are targeted to the site content and audience.
Linkedin Sign-In is an authentication system that reduces the burden of login for users, by enabling them to sign in with their Linkedin account.
Twitter is a 'microblogging' system that allows you to send and receive short posts called tweets.
Varnish is a reverse caching proxy.