The concept of time travel --- movement into the past or future at an unnatural pace --- has been around for quite a while. More recent and far more fascinating is the notion of traveling perpendicular to the usual ticking of the clock.
Imagine another universe, precisely identical to ours except for the position of one electron on one atom in one galaxy billions of light years from us. Now increase that difference, little by little, until the delta amounts to an extra grain of sand on a beach on this Earth (or, more precisely, an otherwise-perfect replica of this Earth). Amplify the change yet further, step by step, to the point where things begin to be perceptibly altered, first just in tiny ways, then more visibly so.
Now string together a movie made from still images taken all at the same time, but with ever-increasing modifications to reality. That's the picture of cross-time navigation that a few science-fiction authors have painted. Among my personal favorites:
Can the once-per-decade pattern of noteworthy yarns in this genre be extended in either direction? Do orthogonal-to-time fantasies tend to be written in series format more often than other sf? And is there a parallel universe to ours wherein with-the-flow time travel stories are exotic and cross time-travel stories are commonplace?
(see also ManyWorldsDemystified (24 Oct 1999), ... )