Here is a source of clisp REPL: http://clisp.cvs.sourceforge.net/viewvc/clisp/clisp/src/reploop.lisp. Usually it's very entangled with low-level implementation-dependent functions of the given CL implementation. I've been playing with ABCL in the same manner to make it run as an applet and it's not easy to create a proper REPL, but when I've found out there exist issues with the java security policy my enthusiasm dropped.
cl-2dsyntax is my attempt to create a Python-like reader. My mirror of CLHS (and the dark themed version). Temporary mirrors of aferomentioned: CLHS and a dark version.