The Main Street USA area music loop is by far my favorite Disney park music and possibly my favorite Disney music to listen to overall.
The albums by The Paragon Ragtime Orchestra "The Whistler and His Dog" and "On the Boardwalk" can be found on the itunes store and I never would have looked for them without my enjoyment of the MSUSA loop. They're both outstanding collections of Ragtime music done with a small orchestra instead of the traditional piano and they go much beyond the regular Scott Joplin standards like "Mapple Leaf Rag", "The Entertainer" etc. They're great is all I'm trying to say.
But back to the loop, the collection of songs from those albums, showtune favorites and obscure Disney music make it so wonderful to listen too. Thank you Mr. Sotto and crew for putting the track list together (even if they re-aranged it for the American parks)!
Other favorites include:
the Spectromagic theme (gotta love the variation on Disney's "Music From the Park" CD, perhas the one good track on that album)
the Welcome and Trolley Shows
Fantasmic!'s theme
Muppet Vision's lobby music
Magic, Music and Mayhem
and lots of others.