OpenSeeIt is a very basic image viewer that can handle a wide variety of formats. In a way, it's intended to work as a replacement for the Windows standard image viewer, although you may use it as a complementary tool as well.
I liked this viewer because it's small, it doesn’t require installation, it works with the most popular graphic formats and features a full-screen viewing mode that lets the user enjoy their pictures in all their beauty, without nagging borders or menus.
Also, there's a Browser view that enables you to easily move around photo folders.
On the downside, I missed slideshows. The Slideshow menu does show up in the program's toolbar, but I didn't manage to get it to work.