Sliders. <3.
I was reading about this in New Scientist pretty recently. From what I gather the problem in parallel universes had something to do with Quantum maths working fine at a specific level, but only being possible at another if the multi-verse theory was accepted. That was generally painted as nuts, but some new work on the maths has revealed that it may not be so crazy after all.
Whether or not the theory works (and in all honesty it's way above my head), it's managed to raise some really interesting philosophical problems. Schrodinger's cat being one.