Find it at GNOME Home
GNOME has a bunch of language bindings to allow developing applications and components in other languages than C. Although ORBit does not yet support Common Lisp, any IIOP-compatible ORB (eg CLORB) will talk to it. Note also that clg and cl-glade will both allow Common Lisp binding for Glade, the GUI construction tool used with GNOME.
GNOME apps use Gtk for widgets and general graphical stuff
There is a "competing" environment called KDE.