http://fxfactory.com/fxfactorypro/
Reveals the destination by pushing the source clip to one side and unfolding the incoming clip to the full frame. The animation can be reversed, thus folding the outgoing clip to reveal the incoming clip.
http://fxfactory.com/misc/FxFactoryHelp/pluginguide/transitions/accordion/index.html
FxFactory is a revolutionary visual effects architecture powering hundreds of plug-ins for Final Cut Pro, Motion, Final Cut Express and Adobe After Effects:
http://fxfactory.com/fxfactory/