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.
OpenSeeIt supports the following formats
BMP, DIB, EMF, GIF, ICO, JPEG, JPG, PCD. PCX, PDD, PIC, PNG, PSD, PSP, RAS, RGB, RLE, SGI, TGA, TIF and a lot more