Answer: As stated by cmassey, "Toxoplasma gondii. Criteria: rupturing cyst with what appear to be escaping tachyzoites. Morphology is crescent shape with a large nucleus and they appear to fit in the 4-8um by 2-3um size given in diagnostic literature."
Great job to everyone who wrote in! This case demonstrates a few nice features of T. gondii. First, there are both cyst and free tachyzoite forms as seen here:
Also, many of the tachyzoites demonstrate the classic 'arc' shape (Toxon is Greek for arc):