How would seasons at the equator be if the Earth were tilted 90 degrees? If Earth's axis of rotation were tilted 90 degrees, what type of seasons would be observed at the equator. 

My prediction is that there would be no seasons, as the orientation of the equator would be the same as if the Earth
I assume you mean "what if the Earth's equator was perpendicular to it's orbit?" in which case the seasons would be greatly exaggerated at the poles and day and night would only really mean something for about half the year if you're nearer the equator. Temperature differential would be much more significant than it is now.

Unless you mean that the axis of Earth's rotation is exactly perpendicular with respect to it's orbit, in which case there would be no seasonal variation whatsoever. (Not including Malkovitch cycles and solar cycles)