Rockman 8 Beta V1: http://www.megaupload.com/?d=6ZGISZMK
(also included in the file, separate mp3s of each unused track!)
The beta spans across two stages: the introductory stage and a vastly different Frost Man stage. Each contains their own set of unused music, different graphics, and other oddities not present in the final retail build.
A set of debug features are readily available for your consumption, too, apparently. By holding down certain buttons, players can move Rockman aimlessly around the screen, slow down animations, and, with a second controller plugged in, control Rockman's stamina. You can learn how to pull that off and more right here!
As for any behind-the-scenes stuff such as unused sprites and the like... nothing's turned up yet.
I've taken a quick peak, but the only hidden elements I could find were some scratch pad notes left by the developers -- handwritten numbers and kanji indicating what goes where. Nothing earth shattering, but still kind of cool.
If you've got the capabilities to dig through the game yourself, please let us know if you unearth anything significant!