It varies every day and at every hour.
On our last visit in March 2003 we found Stitch in adventureland area, Mary Poppins and Penguins on Main St, Eyeore on main st , goofy, pluto, mickey , minnie, rafiki at opening. Buzz in tommorrowland, Huck Finn near River of America (they were white washing the fence- river was drained last yr), Wicked witch from Snow White was outside of Casey the train at a meet and greet, the princesses outside the castle, ariel in her usual spot near the castle.
Other years we have found characters from Pinnocchio, Snow White, Jasmine and Aladdin ( they have a story time in Adventureland with Meet and Greet between shows), Genie, Woody in Frontierland with Jessie, Chip and Dale in Frontierland.
Cruella was easy to find in DCA.
I have always wanted to meet Mulan but I heard she is not around very often.
Others have reported seeing Tarzan and Jane near their tree.