I have had a couple of hands free orgasms triggered by anal/prostate stimulation. I put them in two categories.
The first are the ones that I've had after plenty of foreplay, both anal and genital. These orgasms have been from toys or fingers, either mine or my wife's. They have been mostly of the "OH WOW!" stars and the like kind with moans and the sensations filling the whole lower body. I like these, but then again I also like orgasms with both anal and genital stimulation and even those occasional orgasms where there's been enough play before hand to make it a big fireworks show. Not that I don't like "regular" orgasms, but they are more localized in the genitals and shorter lasting.
The second type are the weird ones. I've had a couple of them - one from a large BM, one from Susie's Very Rough Ride. On the first I was on the toilet then all of a sudden my cock started jerking and I ejaculated. No choirs or stars, more of a "huh? What the?" and I was glad my cock was pointed into the toilet.
The Very Rough Ride handsfree cum was a bit more interesting. I was taking the enema and I guess the mint tingle plus the pressure from the balloon nozzle was too much for my prostate. All of a sudden I just came out and said "fuck, I'm going to cum" and then I did. There was a little of the nice feeling, but a lot of the "huh?" as well, though perhaps the intense rectal stimulation was a factor there.
I'd love to have some more, especially ones from fisting and pegging (and perhaps an enema-driven one), but haven't had the time to do much extended buttplay recently (and not the inclination either, as I'm not feeling so good at the end of the day).