Some of my favourites -

* Enlive, a HTML scraping & templating library implemented on top of DOM parsing & state machines https://github.com/cgrand/enlive

* Core Logic, a Prolog-like logic programming library https://github.com/clojure/core.logic

* The ClojureScript compiler, a Clojure compiler that targets JavaScript https://github.com/clojure/clojurescript/blob/master/src/clj...

* Ring, a Rack/WSGI like web-app library https://github.com/mmcgrana/ring

* Midje a very powerful test framework https://github.com/marick/Midje

* Carmine, a Redis client lib in Pure Clojure https://github.com/ptaoussanis/carmine/

[EDIT] Added Midje, Ring.