XnView includes many special features that you may not find on similar programs, such as its support for almost any image format and multiple languages. And it's completely free.
It also has a very simple and convenient interface that's quite similar to that of the popular ACDSee.
Even though the program is free, it's very comprehensive and the way it works also resembles ACDSee. Furthermore, it supports a wide range of formats and languages.
In addition, for greater ease and portability, XnView now comes in a portable version that can run on any PC without having to be installed on each system.