It is a CLIM application. Its development resources are summarized at its project page on common-lisp.net.
As of July 2006, Climacs is a useful tool for writing Common Lisp code, but it does not have close integration with a REPL, debugger or inspector like SLIME. The CLIM-desktop project provides additional integration with Climacs into other applications (most notably the McCLIM Listener), but does not greatly enhance Climacs' capabilities for editing Common Lisp. Climacs can presently do parameter hinting, code evaluation, symbol completion, displaying of compiler notes and definition-editing.