Disney owns all the characters it uses plus a whole network. The most recent example is House of Mouse, which has Mickey and Co. as hosts of a nightclub/theater, with the characters of the feature films as the audience.
Many classic Hanna-Barbera characters have been used across the board, in shows like Laff-A-Lympics and Yogis Gang; this is still done, but with a more satirical bent (e.g., Harvey Birdman, Attorney at Law). The largest gathering of characters, however, came with a Hanna-Barbera 50th anniversary special in 1989 as all the characters gather together to pay tribute. This doubles as Roger Rabbit Effect as they interact with Joe and Bill themselves, along with special hosts Tony Danza and Annie Potts.
Nearly all recent Warner Bros. animated television series made references and overt cameos with one another.