It will depend upon what you want to do with the images. The nearest thing to Photoshop is GIMP:
http://gimp-win.sourceforge.net/stable.html
(The only problem with GIMP is that, like Photoshop, it takes a long time to learn how to get the best from it)
Irfanview is also popular. It's mainly known as the best free image
viewing program but it's got plenty of useful image manipulation options available as well:
http://www.irfanview.com/
(Irfanview is more basic than GIMP but much easier to learn how to use)
Google's Picasa has got plenty of fans. (It's particularly useful if you want to create online galleries of your images, because it integrates directly into the Picasaweb site):
http://picasa.google.com/
(I've not tried it myself, but the reviews suggest that it's very intuitive).
For many people though, the best free program is Serif Photoplus 6. It's a full commercial program but it's just not the latest version. Most people find that they don't need the 'bells and whistles' of the later versions, so Version 6 is fine for their needs:
http://www.freeserifsoftware.com/software/Phot oPlus/default.asp
(I love all Serif software. It's all easy to use and, with so much of it being free, it can't be beaten for value!).
Chris