Anonymous leaking is an ancient art and many websites publish documents from sources they cannot identify. What Wikileaks has done is to professionalise the operation. They have created a standard procedure for receiving, processing and publishing leaks.
Steven Aftergood of the Federation of American Scientists' (FAS) Project on Government Secrecy — reported in Marks, Paul (May 10, 2008). "A fail-safe way to embarrass people in high places: Whistle-blowers can tell all without being traced, thanks to websites that anonymise their details". New Scientist (Reed Business Information): p. 28, Volume 198; Issue 2655.