IntelliJ IDEA is our polyglot IDE of choice. We use it esentially for all programming and coding aswell as some basic database management and planning. IntelliJ IDEA has proven to be a reliable companion in developing our various software.
Shouldn't surprise anyone, as minecraft is also java-based. Java is used for much more than just the plugins though. JCVortex (our API) is also served with vert.x (Java) and many of our team-internal tools also originated from java or are still java-applications.
Stack overflow HAD to be included in this list. We can't count the endless hours stack overflow and its huge community has already saved us and will potentially save us in the future. Stack Overflow is the proof that nearly every problem one may experience, another one has already solved.
As the basis of our new infrastructure, we use CoreOS as a reliable solution for our docker-server-instances. We plan to deploy all our servers as individual docker containers to make use of the extensive possibilties offered in terms of isolation, resource-managemant (cgroups) and scalability.
We use Postman in conjunction with our universal REST-API "JCVortex". Postman makes testing edge-cases hassle-free and lets testing look easy. Postman was also a great help to explore the Mojang-API, that we are dependent on, because it is the central repository for minecraft-account-data.
jQuery is only used in small amounts, primarily for animations and UIs, but it is included in the WSC, so we felt like not including it here would be kind of cheating. jQuery also almost makes ajax-requests a pleasure to work with, so ... you got that point, jQuery.
The first certs we got issued were EV SSL certificates from Comodo, but we later decided that we do not want to support the whole CA-thing anymore as they constantly get abused and take tremendous amounts of cash in exchange, so we work with Let's Encrypt since over a year.
We first used slack and switched to Discord later to stay near at where the community is at, while still be able to have private conversations and stay in contact. Discord offered everything we needed and used from Slack previously, plus the community-part, so it was an easy decision.