My knowledge is from 2007, so it may be slightly different this year. The 7 dwarves were meeting over by Dumbo. There was a CM who directed everyone into a line before they came out. It was pictures only and there wasn't a photopass photographer.
Villains meet at the end of the Villains Mix and Mingle show in front of the castle. It is a short meet and greet (maybe 10 minutes?) so you may only get to see one or two, depending on how popular they are. That is also pictures only with no photopass photographer. I know you could meet Maleficent, Cruella, the Evil Queen from Snow White, Frollo, Jafar, Captain Hook, and there may be more that I can't remember.
Other characters: Princesses and their princes in the Hall of Fame (I remember seeing Snow White, Cinderella, and Sleeping Beauty), characters at the dance parties in Fantasyland (a tent behind Dumbo) and Tomorrowland (inside Cosmic Rays?). The dance parties have characters alternate so you may see different characters at different times. Dance parties are technically not a meet and greet, so you can't really take posed pictures with them or get autographs, but they are fun. I know Stitch was at the Tomorrowland one. Chip and Dale are sometimes at the Fantasyland one.
The fab 5 do meet and greets in costumes, but I don't know exact locations. Mickey and Minnie will probably be in Toontown in the Judge's Tent.
Someone else may chime in with more, but that is what I remember.