There was an interesting article on hybrid fruit growers last fall in the Wall Street Journal. The article mentions Zaiger's Genetics, Bradford Farms, and Kingsburg Orchards. Although the article doesn't mention him, Luther Burbank was the grandfather of them all.
You can read the article here: When Apricot Met Plum . . .
The article mentions that Zaiger's is close to releasing a Pluerry -- a cross between a plum and a cherry. However, I have not seen it marketed for sale anywhere, so I am wondering if it is still in development.
I am curious as to whether it will be a winner or not.
There are several Cherry Plum varieties on the market, already. I have not tasted them, though. They are advertised to be more bush-like than trees, producing smaller plums. Although they are not originally native to North America, I am curious if they are similar to Beach Plums - another native variety. I have heard from at least one of you that you are growing some of these. Womack Nursery in Texas sells the Hiawatha variety.
Dave Wilson Nursery also sells a couple of varieties of Cherry Plum hybrids which are listed as crosses of plums and cherry-plums - Delight and Sprite.
The following is another article I found listing out several different varieties of Plum-cherry crosses; however, it does not mention Hiawatha: Cherry-Plum Hybrids.
Finally, here is another article on Zaiger's Genetics: Zaiger Article.
If you are looking for a resource that lists all -- or at least a lot of them -- of the various plum hybrids and plum varieties that are available, a good starting place is Plums of North America by Richard Ashton. Nevertheless, given that these breeders are cranking out new varieties every year, it is hard to keep track of them all, much less, catalogue them in a book.