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?
I really like Safari's Web Inspector Javascript console.  Specifically:

Collapsible interactive object hierarchies
sprintf-style logging
Pretty-printing of closures, allowing you to peer into the internals of anonymous functions
Auto-complete / hinting of object properties on the command-line
Slickest REPL console in any language