nomacs is a free image viewer, which is small, fast and able to handle the most common image formats.
Additionally it is possible to synchronise multiple viewers.
A synchronisation of viewers running on the same computer or via LAN is possible.
It allows to compare images and spot the differences (e.g. schemes of architects to show the progress).

The best Sqlite3 GUI tool for developers
in the world. No joking here (or just a bit only) - it
contains the most complette feature set of all tools available.

Bleeding edge trunk...