Warner Bros. Animation's shows like Tiny Toons, Animaniacs and Freakazoid! have the fourth wall breaking a lot. Freakazoid appearing in a obvious 1995 of Animaniacs, Animaniacs and Pinky and The Brain appearing in the Freako Universe. Tiny Toons in the Animaniacs universe, all of it. Hell, even Batman in silhouette form appeared in a Freakazoid! episode.
Both Launchpad McQuack and Gizmo Duck appeared in both DuckTales and Darkwing Duck. Crossing over to another universe.
The Simpsons and Futruama makes fun of Fox (which broadcasts The Simpsons and broadcasted Futurama), and its parent News Corporation, ultimately breaking the wall. Bender appeared two times on The Simpsons in 2000 and 2005. Peter Griffin appeared in a 2002 episode (Treehouse Of Horror segment Send In The Clones).
The 1994 Spider-Man series had a lot of breakings, in a kind of good way. Mentioning the Fox Network, The Tick, X-Men, making references to Eek! Stravaganza. Speaking of Eek!...
While the first two seasons of Eek! The cat and the first season of Thunderlizards were original no wall breaking, once the retooled Eek! Stravaganza came in 1994, the third season had chock full of wall breaking, like mentioning Melrose Place most of the time, mentioning Fox Kids a couple of times, Nelvana also, Eek appearing in Thunderlizards Universe, Thunderlizards in Eek's world, but it was still good. Except for the ultimate breaking, which was Klutter appearing in 1995.