An alternate binding for slime's slime-documentation-lookup-function. Uses l1sp.org if invoked with a prefix arg.

gistfile1.lisp

(defun slime-hyperspec-plus-l1sp-lookup (symbol-name)
  "The usual hyperspec lookup, but with a prefix arg uses l1sp.org instead"
  (interactive (list (let* ((symbol-at-point (slime-symbol-at-point))
                            (stripped-symbol 
                             (and symbol-at-point
                                  (downcase
                                   (common-lisp-hyperspec-strip-cl-package 
                                    symbol-at-point)))))
                       (if (and stripped-symbol
                                (or current-prefix-arg
                                    (intern-soft stripped-symbol
                                                 common-lisp-hyperspec-symbols)))
                           stripped-symbol
                         (completing-read
                          "Look up symbol in Common Lisp HyperSpec: "
                          common-lisp-hyperspec-symbols #'boundp
                          t stripped-symbol
                          'common-lisp-hyperspec-history)))))
  (if current-prefix-arg
      (browse-url (format "http://l1sp.org/search?q=%s" symbol-name))
    (slime-hyperspec-lookup symbol-name)))

(setf slime-documentation-lookup-function 'slime-hyperspec-plus-l1sp-lookup)