Our app is built using Ruby on Rails. We deploy on Heroku via github. We have separate instances for prod and staging, and can increase/decrease dyno's depending on load.
All our transactional emails go via Sendgrid, so account created, test started, test finished etc.
iOS and Android apps are stored in S3, as well as icons that we extract out of the binaries
our main database is a MySQL RDS instance with. Schema changes are managed by rails migrations
for source control and issue management we use Github. We use git as well to push code out to Heroku