Common Lisp

By the early 1980s there were Maclisp, Interlisp, Zetalisp, Spice Lisp and NIL, all mutually incompatible, and DARPA had no appetite for funding five parallel ecosystems. The result was the book Common Lisp the Language in 1984 and eventually the ANSI standard in 1994 — one of the most thoroughly worked-out language specifications ever written, and one of the largest.

What makes the language unusual is not the parentheses but three things: macros that run arbitrary code at compile time and therefore let you grow your own syntax, CLOS with multiple dispatch and method combination, and a condition system where the handler can decide to resume from the point of the error rather than unwind the stack. Still very much alive: SBCL is fast, and anyone who has debugged inside a live image tends to find every other language primitive afterwards.