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By aj_s:
http://www.minecraftforum.net/topic/736138-very-compact-bidirectional-shift-register/
(This one has pistons)
By me:
http://www.minecraftforum.net/topic/672807-vertical-bidirectional-shift-register/
By Grizdale:
http://www.minecraftforum.net/topic/676498-grizdales-bi-directional-shift-register/