Brightly Colored Flat-Backed Millipedes (Xystodesmidae)

 Apheloria virginiensis is perhaps the most common one, but there are others in NJ. 

Greenhouse Millipede (Oxidus gracilis)

 not entirely round, with a groove cross-wise on each segment

American Giant Millipede / Worm Millipede (Narceus americanus/annularis complex)

 Can't separate the two species (N. americanus and N. annularis) so it's a "complex". These are the huge, red-striped millipedes.

Round-Backed (or Snake) Millipedes (Julida)

 In NJ there are two main families: Julidae (invasive) and Parajulidae (native). I cannot separate them.