Cusp is a powerful IDE for Common Lisp that works right out of the box. It is similar in functionality to Slime, but without the design flaw of being in Emacs.

Cusp is an Eclipse plugin, which means it is written in Java. It uses SBCL as its Lisp implementation. It is open source. It is shiny.

Please note: It is called Cusp, not CUSP. We use it to program in Lisp, not LISP.