FastStone Image Viewer
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Full user review
"Great photo editing program."
Easy to use. Lots of features. Opens RAW images. Good image straightening/cropping routines. Gamma adjustment can do a quick save on underexposed images. Easy to do a Print Screen grab, crop, resize and have something to use in a post.
Can't think of any. Obviously it's not Photoshop. But it is free vs. several hundred dollars.
(Do contribute if you use it.)
I'm using FS for my routine editing - cleaning up lots of images for slide show/web use. I might dive into a deeper program when making a critical print, but FS does the normal stuff very well.
Irfanview is my choice for building quick panoramas (non-overlapping frames) and for organizing into folders.