Unofficial flags are either (a) flags in regular use that have no official status or (b) flag concepts that have appeared in various publications.
Examples of the former type are: Boxing kangaroo, Fighting Kiwi, Chatham Islands, Rotuma, West Irian, Jolly Roger. These are examples of flags used at sporting fixtures, reproductions of no-longer-used historical flags, or used by a regional populace out of spontaneity, though not officially approved (or gazetted).
Examples of the latter type are: North Island, South Island, Stewart Island/Rakiura, 51-Star United States Flag, Antarctica. These are examples of flag ideas put forward by vexillologists/flag designers that have been picked up by various publications (books, magazine articles, online reference sites, etc.).
Chatham Islands flag (used widely, but status unofficial)
Unofficial local flag of Saint Pierre and Miquelon
Flag of Wake Island used in military ceremonies
Korean unification flag, used in sports contexts
South Island (NZ) conceptual flag
51-star conceptual United States flag
Flag of Johnston Atoll (currently an uninhabited U.S. territory)