Scooby-Doo appeared twice in The Grim Adventures of Billy and Mandy, in the episodes Reap Walking and Keeper of the Reaper, the former was a non-speaking cameo with the rest of the gang, while the latter he was voiced by Welker.
The gang were guest stars in Bravo Dooby-Doo. Welker, Kasem, and North reprised their roles as Fred, Shaggy, and Daphne, respectively. While new actors B.J. Ward and Hadley Kay were hired for Velma and Scooby, respectively. Kay then reprised the role for a Scooby cameo in 'Twas the Night.
In Sabrina Unplugged, when Sabrina is caught by the office webcam using her powers, she magically enters Leonard's computer to destroy the evidence and meets up with Shaggy and Scooby (voiced by Innes and Welker, respectively). He asks her if she can get them off as a screensaver, as they're embarrassed. Both shows aired on The WB at the time.