The other thing Cross-Universe means to me is also known as Paratime, Otherwhen, Alternate History, and the Multiverse. The list from this group would take MANY entries, and is my favorite form of ‘What If’ entertainment. My introduction to it was through H. Beam Piper’s Lord Calvin of Otherwhen , a serious classic I had reread 5 times before I found my next example. For more detail on Piper’s stories, and a wonderful Sideways In Time site, be sure to check out UChronia’s Piper Page.

A few others include James P. Hogan’s Paths To Otherwhere , Joan Aiken’s Wolves series, anthologies such as Roads Not Taken , game universes like Steve Jackson and GURPS, and way more than I can go into here.

Eric Flint and Harry Turtledove are building some of my favorite new stories in this category; the history of alternate history goes back to Ben Disraeli in the early 1800’s, and Winston Churchill (at the same link) in 1931. I don’t find it a great surprise that the folks who lived their lives at the pivot points of history were some of the earliest contributors to this genre!

And drop by Alt.History , just for a grin.