Given that it's on this side of the mount (passenger side), the FIRST thing I'd look at would be the valve cover gasket, and the cam plugs(since a plug lead could bead down the seam of the VC gasket and drip over there).
EDIT: While you're in there though it'd be good to check the rest of Larry's list. On my car, all of the above were leaking to some extent or another when I bought it.