There is something called nutritional yeast. It seems that it is generally used as an additive to recipes or as a seasoning.
If one ate this, plain, what bracha should he make prior to eating this? According to this site, yeasts are fungi, so I assume that the bracha would be shehakol - similar to mushrooms. However, from what I could glean from my searches, there doesn't appear to be a habit or suggestion to eat plain yeast, and it seems to always be used in something else. Thus, it's possible that it may not be considered a "food" and perhaps requires no bracha at all.