Producer is a term that describes anyone who creates their own original songs. Some are using the VOCALOID software. Those are usually identified in the community by the honorific "P" after their nickname (e.g. RyuuseiP). However some also create original songs. On Nico Nico Douga they're commonly referred to as Nico Nico Indies (NNI).
In rare cases, producers can also create PV's or artwork for their own works.
The following is a list of utaite who are also producers. Some also do self-covers of their own produced works.