Okay, Karl. Here is my secret. Its name begins with Irfan, because Irfan made it, and ends with View, because it's an image viewer. But it is also much more than that. In fact, it's one of my favorite pieces of software ever in terms of simplicity, speed, and power. But definitely not in terms of logo design. (Sorry, Irfan, but I'm just not sure if "bright red roadkill burglar cat with a magnifying glass" accurately conveys the coolness of your app. On the other hand, now that I see it written out, it does have a certain je ne sais quois...) Loading instantly and taking no memory is always a good first step.
Some things you usually want to do when browsing and editing a folder of photos:
1. Press Enter to go in and out of showing the photo full screen. Use the + and - keys to zoom in and out when not in full-screen mode.
2. Whenever a photo is open, simply use the scroll wheel or arrow keys to go between photos in the same directory. (These may also pan if the image is larger than the screen, but you can disable that.) There is a slideshow mode to flip through images automagically.
3. To delete the photo from your computer, press Delete. (This is, sadly, pretty revolutionary.)
4. It's not just a viewer. You can adjust contrast, gamma, hue, saturation, etc. in the "Enhance Colors..." dialog. In fact this is where I do most of my photo editing. It also works with Photoshop plugins.
5. You can also Crop (Ctrl Y), add a border (Canvas Size...), Resize/Resample (Resize/Resample (Resize/Resa... oops))), and so forth.
6. Set the jpeg output quality to about 95, and your filesizes will likely be two thirds or less of the camera's original output filesize, with little to no loss in image quality. Which is especially handy when you can...
7. ... batch edit a whole folder of files. Simply resaving with no changes is an excellent method for saving hard disk space, but you can also do all the other things mentioned above, resulting in a whole folder of resized, bordered, sharpened, corrected photos. So there.
8. And more! Lots of settings and customizability, as well as other useful functions such as the robust screen capture abilities used for the above picture, and the ability to display non-images such as pdfs, movies, or flash files, though I usually set it to only show images for the sake of simplicity.
9. Finally, be sure to get the plugins, which are handily all available as a single download. One of them will let you view EXIF data within a JPEG. (Press "i" when viewing an image to open its info, and then "e" to view the EXIF.) This data usually includes camera specifics such as model, shutter speed, aperture, ISO, date/time, and, if the picture is of a person, what that person looked like at the time of the photo. All useful tools for spying on the internets.