The Clojure debug-repl 
Every time I stick a println into some Clojure code to debug it, I
think to myself, "This is Lisp! I should be able to insert a repl
here!"
The problem is of course that Clojure's eval function doesn't know
about the surrounding lexical scope.  How to solve the problem?
Create a macro that passes a copy of the lexical scope in with the
form to be evaled, something like this:
(defn eval-with-locals
  [locals form]
    (eval
     `(let ~(generate-local-bindings locals)
        ~form)))
USE:
The interface is meant to be dead simple:
"(use 'alex-and-georges.debug-repl)" loads it, and "(debug-repl)" invokes it.
That's about it.  When you enter the debug-repl, the regular
repl prompt will be replaced with 
dr =>
An example will make it clearer:
user=>   (let [c 1 d 2]
    (defn a [b c]
      (debug-repl)
      d))
#'user/a
user=>   (a "foo" "bar")
dr-1-1001 => c
"bar"
dr-1-1001 => d
2
dr-1-1001 => *locals*
{fn__104 #<user$eval__103 user$eval__103@5f6303>, c "bar", d 2, fn__106 #<user$eval__103$a__105 user$eval__103$a__105@179dce4>, b "foo", counter__56__auto__ 1001}
dr-1-1001 => (str b c)
"foobar"
dr-1-1001 => ()
2
user=> 
LIMITATIONS:
The debug-repl doesn't currently integrate properly
with the slime-repl, (I think because of how Slime manages IO
redirection,) so you'll have to invoke it from a regular repl, or
slime's *inferior lisp* buffer.
A version of the debug-repl has been ported to slime.  More details
here:
https://hugoduncan.org/post/2010/swank_clojure_gets_a_break_with_the_local_environment.xhtml