FreeVimager is an open source image viewer which offers far more functionality than you'd expect from this kind of tool.
There's all the basics, of course. The program can associate itself with common image formats, so you can view them with a quick double-click from Explorer. Or you can open a folder of pictures, then browse them in a customisable slideshow. And there are plenty of editing options: rotate, resize, crop, sharpen and soften effects, a red-eye remover, brightness/ contrast/ colour tweaks, and more.
But many of these functions go further than we expected. FreeVimager doesn't just display your images in slideshows, for instance: it can burn them to CD or DVD. The program keeps metadata (EXIF, IPTC, XMP, ICC) during its edits. Many editing options are surprisingly configurable for an image viewer (the Resize option offers 5 resizing methods, for example), and there are some tools which you won't even find in many commercial editors (lossless JPEG cropping, or the ability to add, update or remove an EXIF thumbnail).
And there are more than enough bonus features to justify downloading the program, all on their own. Like its ability to save images as PDF files. The scanning tools (scan multiple pages to a PDF or TIFF file and email them, for instance). And a particularly powerful batch processing dialog can take a folder of images and convert or rename them to suit your needs.
The program is also a little basic in some areas. Image format support is essentials only (bmp, emf, gif, jpg, pcx, png, tiff), and there's no thumbnail browser.
On balance, then, FreeVimager is an excellent image viewer, powerful, portable and with something to offer just about everyone. Go grab a copy right now.
Note this is the portable version of FreeVimager.