OpenSim Creations have a lot of resources, ranging from full-structures to clothes and miscellaneous items.
Zadaroo Public Domain OpenSim Resources
Outworldz.com - a compendium of OpenSim resources and for Second Life too.
OpenSim HyperGrid List - a list of publicly accessible grids for all tastes.
Wizardry and Steamworks offers a collection of scripts and other collected resources such as particles (in the Second Life resources folder).
TF3DM The Free 3D Models has free mesh objects in several formats. Lots of superheroes.
highend3D 3D models but not all free
Flat Pyramid Royalty Free multiple file formats.