Sulfur Trioxide - Ionic Character
Sulfur trioxide is planar with 120 degree O-S-O bond angles.  If the molecule were unhybridized and we used the 3 orthogonal p-orbitals for our sigma bonds, then we would expect the O-S-O angle to be 90 degrees.  Further, in the unhybridized scheme, there are not enough s orbitals to form 3 sigma bonds that are pure s. Just considering s and p orbital involvement, we could say that the sulfur is $\ce{sp^2}$ hybridized.  This would produce 3 $\ce{sp^2}$ orbitals for sigma bonds from sulfur to oxygen and leave one p orbital for one pi bond.  We can draw 3 resonance structures for this arrangement, so we would expect all bond lengths to be equivalent.