my-let, from Alex P., uses functions to implement a version of the Clojure special form let:

(defmacro my-let
  [bindings & body]
  (assert (-> bindings count even?) "Bindings count can only be even.")
  `((fn [~@(take-nth 2 bindings)]
    ~@(take-nth 2 (rest bindings))))

This isn’t a complete implementation of let, since it doesn’t allow bindings that depend on previous bindings, like in (let [a 1 b (inc a)] ...). And because it’s implemented in terms of functions, it also allows the let to be a recur target (just like loop). It’s pretty cool how little code it takes to write a feature that can do this:

(my-let [{:keys [a b] :or {a :foo}} {:b 2}]
  [a b])
;=> [:foo 2]

Leaning heavily on pre-existing language features can be a huge help.

Check out Alex’s nice and succinct explanation of the macroexpansion for more detail.