We initially chose RethinkDB because of the schema-less document store features, and better durability resilience/story than MongoDB In the end, it didn't work out quite as we expected: there's plenty of scalability issues, it's near impossible to run analytical workloads and small community makes working with Rethink a challenge. We're in process of migrating all our workloads to PostgreSQL and hopefully, we will be able to decommission our RethinkDB deployment soon.
We use collectd because of it's low footprint and great capabilities. We use it to monitor our Google Compute Engine machines. More interestingly we setup collectd as StatsD replacement - all our Clojure services push application-level metrics using our own metrics library and collectd pushes them to Stackdriver
We use Clojure because of simplicity, performance and great fit to our problem domain: processing a lot of structured data. Based on my experience so far, Clojure is my go-to language for most tasks, except for when a fast startup time and a low resource footprint is required - basically CLI tools. That's slowly starting to change thanks to native-image and GraalVM
Ruby is the original language at Enjoy: the prototype was built with Ruby and some of its parts are running till this day (I'd say 80% of it is gone, ported to Clojure)
Outside of usual web-development tasks we use Ruby for our operations tooling.
We use nginx because it's super reliable, has great extensions and at the same time is light-weight. nginx powers our authorization layer, inter-service communication, and public load balancing (fronted by Google Compute Engine load balancers).