There's a number of meet-and-greet areas at MGM, many of which feature characters you can't find anywhere else at WDW.
Woody, Buzz, Jessie, and occasionally a Green Army Man or two appear daily at Al's Toy Barn right across from the exit of MuppetVision in the Backlot.
Chicken Little and Abby Mallard have appear inside the Magic of Disney Animation exhibit at the Animation Courtyard.
Mike and Sully from Monsters Inc. appear near the ABC Commisary on Commisary Lane (just look for the garage door with the Monsters Inc. logo on it)
JoJo and Goliath from Playhouse Disney have their own trailer on Mickey Ave. near Who Wants To Be A Millionare.
A number of Power Rangers appear on the Big City Street near the Lights Motors Action Entrance.
Mickey appears dressed in full Sorcerer's Apprentice regalia at Soundstage 1 on Mickey Ave. (near the entrance to the Backlot Tour).
There's also a number of characters like Minnie, Pooh, Stitch, etc. that appear in front of the Hat throughout the day.
Here's a more complete list of who is commonly found there and the other parks:
http://www.wdisneyw.co.uk/greet.html