Krugle gets an automatic win in my book because it seems to be the only one that can search Common Lisp code.
On Google and Koders, setting lang=Lisp turns up mostly Emacs Lisp, and "lang:cl" gives only Tcl code. (They distinguish between CL and Scheme, but not Elisp!) O'Reilly has no language selector, but searching for a Lisp symbol just turns up a lot of Emacs Lisp books. And Ucodit can't seem to find any Lisp code at all.
On Google and Koders, setting lang=Lisp turns up mostly Emacs Lisp, and "lang:cl" gives only Tcl code. (They distinguish between CL and Scheme, but not Elisp!) O'Reilly has no language selector, but searching for a Lisp symbol just turns up a lot of Emacs Lisp books. And Ucodit can't seem to find any Lisp code at all.