All of our frontend code is on AngularJS. Directives, controllers, and services really help in organizing code in order to keep things maintainable, and two-way binding makes data input easy. The large ecosystem of modules for directives is fantastic, too.
All of our pull requests are automatically tested using Jenkins' integration with GitHub, and we provision and deploy our servers using Jenkins' interface. This is integrated with HipChat, immediately notifying us if anything goes wrong with a deployment.
We monitor and troubleshoot our app's performance using New Relic, which gives us a great view into each type of request that hits our servers. It also gives us a nice weekly summary of error rates and response times so that we know how well we've done in the past week.
All of our servers are provisioned by Ansible, with no manual provisioning involved. This lets us easily roll out new servers, with no chance for human error when configuring the servers. We use the same code to provision local development environments on Vagrant, which eliminates configuration discrepancies between development and production.