Cartoon Cross-overs

There was a Ben 10/Generator Rex crossover a few years back. Not to mention "The Jetsons Meet The Flintstones." Plus several episodes of "The New Scooby-Doo Movies" had crossovers with Jeannie, Josie And The Pussycats, and Speed Buggy. Also, Scooby and the gang had crossovers with Dynomutt early on.

But perhaps the greatest crossovers of all time occurred in Space Ghost's "Council Of Doom" story arc. Here, Team Space Ghost was helped out by Mightor, Moby Dick, The Herculoids, and Shazzan.

There was an episode of the Disney series Hercules called "Hercules and the Arabian Night" in which Hercules and Aladdin end up teaming up to defeat Hades and a resurrected Jafar. This takes place before Hercules' movie but after all of the Aladdin ones.
The Lilo and Stitch series also had crossovers with Kim Possible, The Proud Family, American Dragon Jake Long and Recess.
There also was a Powerpuff Girls (The reboot) and Teen Titans Go! crossover. I really don't want to talk about that one as I have no interest in either of these reboots.
After Evil Con Carne got cancelled, one of the characters, General Skarr moved into Endsville and became a recurring character in The Grim Adventures of Billy and Mandy. This made it possible to sneak in Hector and General Ghastly into an episode called "Company Halt" in which Hector attempts to re-recruit Skarr to help him take over the world from his house in Endsville.

There also was The Grim Adventures of the KND which was basically a Grim Adventures of Billy and Mandy and Codename Kids Next Door crossover.
There were also three crossover specials between The Fairly Oddparents and Jimmy Neutron, Boy Genius.