Source Control: Git + Phabricator

Development stack: Ruby on Rails, MySQL, Backbone.js (Coffeescript)

Testing: RSpec, Konacha (continuous integration using Jenkins)

Server: SaltStack for provisioning, nginx/unicorn

Deployment: Capistrano

I think that's a good overview. Then there are a lot more 3rd party services like S3, Sendgrid, etc..