The fruits of several different cactuses are called pitahayas. The yellow ones in the picture are Hylocereus megalanthus, and the pink ones are Hylocereus undatus. To be honest, I was rather disappointed by the (lack of) flavour of them both. Some time ago I had what I think was a Hylocereus costaricensis, which was deep red all the way through, and much tastier.
I wish I had a bigger garden, because this is what Hylocereus looks like: