Via Don Paco, who blogs at Ite ad Thomam, the University of Toronto has posted scanned pdfs of twelve of the first thirteen volumes of the Leonine edition (for some reason, they have not posted volume five). Links to all of the volumes can be found here.  

Speaking of Don Paco’s website, he has scanned an extraordinary number of out-of-print Thomistic resources that he has made available for a small fee.  To see his list, go here.