Slack is a lifesaver, not only for our day to day team communications and it's direct links into our other tools, but for Beta testing as well, with our custom Slack bot in our beta group being an invaluable asset to avoid giving our testers direct JIRA access. Slack
When it comes to provisioning tens to hundreds of servers, you need a tool that can handle the load, as well as being extremely customisable. Fortunately, Salt has held that gauntlet for us consistently through any kind of issue you can throw at it. Salt
When it comes to beta testing on iOS, there's really no other solid option. TestFlight is reliable, easy to use and has a set of great API's (albeit somewhat hidden). TestFlight
Xcode is our primary development platform for iOS applications, with a very fully featured set of dev tools for the platform. For everything else, there's Sublime Text 3. Xcode
GitLab is our main Git server, housed on a separate box inside our VPN, it's diverse features and sandbox-support allows it to be an extremely good way to secure your source code. GitLab