After 5 seasons on Fox, Batman: The Animated Series, which had morphed into The Adventures of Batman & Robin 3 years in, shifted over to WB to join Superman: The Animated Series, which was entering its 2nd season. The network decided to put the two together in a hour-long Saturday block for the first time since 1968, when Filmation held the license and aired the cartoons on CBS.
Because Time Warner had already produced a compilation series under the title, The Batman-Superman Adventures for cable, this block had "New" added to the title to create distinction. As a result, The New Batman-Superman Adventures lasted three seasons before WB pulled the plug. With Superman also airing weekdays, WB had the option of either using a fresh episode on Saturdays, or repeating one from earlier in the week.
From this spun a 90 minute special editon, "World's Finest", which brought the Man of Steel (Tim Daly, ex-Wings) and the Dark Knight (Kevin Conroy) together to battle the Joker (Mark Hamill) & Lex Luthor (Clancy Brown). That particular adventure was reviewed a while back. Bat-fans will point to the episode, "Holiday Knights", as one of the best Batman episodes in this series, one that was adapted from the comics.
Today, the Hub holds the rights to all of the 90's DC toons, running them mostly in the morning to counter-program Nickelodeon, Cartoon Network, et al. For now, let's scope the open, uploaded by Christopher Fluhr: