E.g., it lets you type things like :ld foo to load a file, :exit to quit Lisp, and so on.
Peder Klingenberg has written a similar thing, at http://heim.ifi.uio.no/~pok/download/commands.lisp and there's an ACL-compatible REPL for SBCL included in the contrib directory.
What topic categories should this be listed in? compatibility layers? Possibly Development?