iFactor Pro contains support for complex (imaginary) numbers in addition to a one-of-a-kind Exact Value Mode. Exact Value Mode will keep answers in fraction and radical notation -- it even reduces fractions and breaks down radicals completely!
Factoring: 6x² + 13x + 6 = (2x+3)(3x+2).
Solving (Exact Value Mode): 6x² + 13x + 31 = (-13/12) ± (5/12)i√23
Solving (Decimal Mode): 6x² + 13x + 31 = -1.083 ± 1.998i
This app contains both the iPhone/iPod and iPad versions.