Clojure
Modern Lisp on the JVM. Rich Hickey, 2007. Functional, immutable persistent data structures, concurrency via STM and core.async.
Pragmatic: full interop with Java libraries. ClojureScript compiles to JavaScript. Famous for the "Simple Made Easy" talk (Rich Hickey, 2011). Strong community around fintech (Walmart, Nubank), data engineering, Datomic (Hickey's own DB). Competitor: Scala (more mainstream on JVM), Kotlin (more pragmatic Java replacement).