I like them too - their dark purple look so pretty in the spring garden. The ones that I use here in central NJ (zone 6b) are sweet English Violet - see link: http://americanvioletsociety.org/Violet_Keys/Viola_odorata_002_L.htm

I live in the UK and know that there are meany Viola that are able to grow in clay like soil, the truth is though that to add just one bag of peat to the clay soil will help brake it up, also add sand preferably sharp sand, now back to the best one's to grow in hard soil such as clay soil, and the truth is that almost any should do well however I suggest you grow Viola Freckles as these are perennial ground cover plants that will spread quite fast and are very pretty when in flower, they have very large dark green heart shaped leaves that are around 4 to 5 inch across and from stem to tip, they have large white flowers that look as if someone has splattered purple or blue paint all over them, that's the reason they are called Freckles, they are clump forming and spread quite fast, I suggest that if you do grow them, you only buy two or three plants at most, unless the area you want to cover is a large area, you are able to get these plants through EBay, you should also look for pictures of them first before purchasing any, they grow to about 10 inch in height and their flowers are above the green leaves as a rule, they may not flower in their first years, and the flowers are as big as a pansy.