The best site for actual scouting reports on minor league players has to be Baseball America. There really isn't any free site out there that gives a detailed scouting report like Baseball America. Although, to a certain extent you may look at projectprospect.com.
Here's a basic scouting report on Adam Miller:
Frontline stater w/ 4 seam fastball (93-95/98), 2 seam fastball, changeup, his best pitch is his slider with tilt, late break and power, good command, great makeup.