Many languages of REPL consoles with additional features like autocomplete and intellisense.  For instance, iPython, Mathematica, and PyCrust all make some effort to go beyond a basic read eval loop.  REPLs are particularly useful in languages where interactive exploration is very important, such as Matlab or R.
I'm looking for inspiration.  What application provides the slickest REPL?  Or what features do you always wish existed in your REPL of choice?
Common Lisp and emacs with SLIME. All you could really want, think of, dream of, and then some.
Slickest REPL console in any language