Databases

Barabási, A.-L. and Toroczkai, Z.
Barabasi's network data collection
Set of complex networks studied on works by Albert-László Barabási and collaborators.

NeuroMorpho.org
The largest dataset of 3D digitally reconstructed neurons from over 60 institutions around the world.

Batagelj, V. and Mrvar, A.
Pajek networks data sets
Complex networks files compatible with the Pajek software.