Viking is a multi-user modular viewing and annotation system which functions with any type of stacked 2D imagery registered into a 3D volume. Originally designed for transmission electron microscopy connectome data Viking has been used for confocal, bright field, and OCT images.

Viking provides multi-user concurrent annotation of data sets using a variety of 1D or 2D shapes, including arbitrary polygons. Annotations are stored in a spatial database. The largest current database has well over a million annotations.

A paper describing Viking is here:
http://onlinelibrary.wiley.com/doi/10.11...

Collaborating labs host images, typically registered with the Nornir tools, locally. Annotations are either stored on the Viking servers or optionally labs deploy local back-end servers with assistance from the developer.