IrfanView is a photo viewer and editor that's been around for a very very long time and seen countless improvements since its inception. It allows for the robust viewing of images doing noticeably better than the default image viewer on most operating systems while also supporting the editing (and batch editing) of photos: correcting red eye cropping or adding borders and other common tasks.
Easy to Learn Hard to Master
Over the years IrfanView has become better and better for its intended purposes. When viewing images you can zoom in or out seamlessly and the image will resample to give you the best possible viewing image at any level. You can resize images permanently the same way. You can apply a huge variety of filters or corrections and with plugins to do even more than that. It's IrfanView's interface that's lagged behind the times. The menus are huge and difficult for a new user to navigate.
A Good Balance
As one example moving a single file uses the same interface as moving files in batches and requires you to permanently set the movement directory. Oddities like this aren't bad for power users but do add a small learning curve to the program. But if you just view images none of this matters and IrfanView is perfect. If you edit them and have a little time to learn it's still perfect.