Google's Picasa, as suggested by Rojash, is easy to use and popular.
GIMP, as suggested by Chuck, is the nearest you'll get (without opening your wallet) to the flexibility of Photoshop. But, just like Photoshop, you need to spend a lot of time, learning how to use it, in order to get the best from it.
Irfanview is the best free image
viewing program available (and deserves a place on everyone's PC). It also offers some good image editing facilities. (It's particularly good at getting rid of red-eye):
http://www.irfanview.com/
Serif is a great commercial software company because it makes the older versions (or 'light' versions) of some of their programs available for free. Since most users don't require the 'bells and whistles' of the latest versions, that means you can get hold of full commercial software totally free of charge. Serif PhotoPlus SE is well worth a try:
http://www.freeserifsoftware.com/software/phot oplus/photoplus-9.asp
Chris