Chartbeat (http://chartbeat.com) - Realtime analytics
Sendgrid (http://sendgrid.com) - Sending email from an app
Dropbox (http://dropbox.com) - File sharing/storage
CouchDB (http://couchdb.apache.org) - Flexible database
1. Our users can send messages to us from inside the app. They end us using this to communicate with us more than email.
2. The dashboard for site owners is awesome. It shows you overall user count, who is logged in right now, each user's social networks (pulled automatically using http://www.fullcontact.com), and gives you a whole bunch of ways to filter the information.