A Dariaverse crossover is exactly what it sounds like: a fanfic in which Daria is crossed over with Daria. Characters from one Dariaverse (usually very close to canon) somehow make their way into another Dariaverse (almost always an alternate version of the canon setup), and chaos typically results. The earliest known Dariaverse crossover was "The Sun Will Come Out, Tomorrow" by Brother Grimace (June 2000), with a scene of Daria visiting a parallel universe; the second oldest, "Through a Closet, Darkly" by Brian Taylor (November 2000), was entirely devoted to the crossover between 'canon' and a darker alternate. A later but memorable story was "You Only Hurt the Ones You Love," by Crusading Saint.
The idea has grown to include characters from one author's Dariaverse going into one or more other authors' Dariaverses. The 2005 story "Illusions," by CharlieGirl, Angelinhel, and The Angst Guy, presented the idea that an ancient alien race called the Architects had invented a large number of inter-universal gateways using their godlike technology, and one such gateway found its way into the back of the Good Time Chinese restaurant in Lawndale, where characters used it to explore other realities. This idea was seized upon in later stories: "Three" and Worldburner fic "When It Rains" by Richard Lobinske; "Second Chances" and "End of the Line," by Lycissa; and "Guys' Night Out," by TAG.
An interesting combination of a Dariaverse crossover and an alternate history world is The Sidhe's "A Yankee Marine in General Lee's Army." Another interesting variation, in which all possible Dariaverses are found to be linked together through one nexus, is "In Dreams," by Scissors MacGillicutty.
The Tales of the Ringbearers series makes extensive use of Dariaverse crossovers, many of them original though a few are borrowed or modified versions of previously existing Dariaverses. The Judith Strikes/Worldburner/Worldhopper series are one large Dariaverse crossover, on an epic scale.
Compare to reality crossover.