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It depends on what you mean by Parallel Worlds. If you mean worlds where events have moved in ways that are different to our timeline through actions of a villain or the Doctor then the list is quite extensive.

If you mean timelines that are alternate and independent of our own then the list is smaller. "Inferno" saw the first such trip by the Doctor (in his third incarnation) where he travelled to an alternate Earth. The Tenth Doctor travelled to the Pete's World in "Doomsday" and in the Cybermen two parter earlier in the second series.