Beast is a Basic Entity/Aspect/System Toolkit for Common Lisp. It's a thin layer of sugar over CLOS that makes it easy to write flexible objects for video games.
Beast can be installed with Quicklisp: (ql:quickload :beast)
Check out the Overview for a three-minute description of what this is, or the Usage for a full rundown.
The test suite currently passes in SBCL, CCL, ECL, and ABCL on OS X and Debian. Further testing is welcome.