Answer: Cystoisospora (formerly Isospora) belli
As pointed out by the readers, these are immature oocysts, since only 1 sporoblast is visible.
This is the state that this organism is shed in the stool of infected individuals. The oocysts then mature in the environment into their infective form, with the sporoblast dividing into 2 sporoblasts. The sporoblasts develop cell walls and become sporocysts containing 4 infective sporozoites each.
Thanks to Florida Fan for pointing out that the modified/hot safranin stain will also beautifully stain the oocysts of Cystoisospora (and other coccidia). The oocysts also exhibit striking autofluorescence using UV microscopy (excitation filter at 330-365 nm or 450-490 nm).
I hope you all had a safe and happy July 4th weekend!