Irfanview is still the king of image viewers

Since I first started using the great free image editing software that is Irfanview way back in 2001, it has not taken a backwards step and has only continued to improve.

I refuse to use anything else. It is fast, easy and intuitive.

I find the bulk image conversion functions to be particularly useful and it supports almost any format (handling its file associations particularly well). While it isn't an image editor, it is fantastic for quick and easy viewing of images.