JPEGView is a small (798KB download) image viewer which is absolutely crammed with interesting features.
The program supports a good range of file types, including some RAW formats: JPEG, BMP, PNG, TIFF, GIF, WEBP, WDP, HDP, JXR, PEF, DNG, CRW, NEF, CR2, MRW, RW2, ORF, X3F, KDC, NRW, DCR, SR2 and RAF. And it loads and processes them quickly, too, thanks to its support for SSE2 and multiple cores.
Once open, you can open and resize an image by spinning the mouse wheel, or using a hotkey (there are keyboard shortcuts for every mouse action). And a small control overlay at the bottom of the window provides various useful options. You can step backwards and forwards through images in the same folder, for instance; switch to a full screen or "actual size" view; rotate images by fixed increments or just by dragging the mouse (an on-screen grid helps you get it aligned); and there's perspective correction, auto image enhancement, and a simple EXIF viewer.
JPEGView would be useful if it only did this - but the program is just getting started. Move your mouse cursor below the control overlay and more options should fade into view (if they don't, maximise the window and try again). A collection of sliders allow you to tweak contrast, brightness, contrast, colours (CMY/ RGB); there are options to tweak your shadows, and highlights, correct colours and contrast; there's an Unsharp Mask option, and you can even give the image a new name.
Still not impressed? Then right-click the image for even more. Like the option to display your images as a slideshow or movie (that is, at speeds of anything up to 100 per second); a complete batch file renamer/ copier; and a settings file which gives you more options, including the ability to extend JPEGView with other programs and custom commands.
With all this power, you might think JPEGView is going to be confusing - but it really isn't. Most of the more advanced options are hidden away, so on launch you're only presented with a basic navigation bar, simple enough for almost anyone to use. But experienced users can then easily explore further (hint: start by pressing F1 for a lengthy list of hotkeys).