FastStone Image Viewer (also freeware). If you want, you can slip them a small, optional donation (which I did - I believe they take something like a fixed $10).
Although I have others image programs, I use FastStone most of the time. Unfortunately, it doesn't have a 'clone' tool - and you need one if you want to repair photos.
Gimp comes well-recommended (and has a clone tool). But, as others have said, "it has a steep learning curve", ie it's a bugger to use until you you've spent a lot of time mastering it. One day, I might get the hang of it!