The GIMP ~ Gimp.org
*****
Available for: Windows Vista/XP/NT 4/2000
Download: home page | Download.com | FileHippo

Paint.Net ~ Rick Brewster
****
Available for: Windows Vista/2003 Server SP1/XP SP2
Download: home page | Download.com | FileHippo

Picasa* ~ Google Inc.
****
Available for: Windows Vista/XP/2000
Download: home page | Google Pack | Download.com | FileHippo

FastStone Image Viewer ~ FastStone Soft
****
Available for: Windows Vista/XP/2000/98
Download: home page | Download.com | FileHippo

IrfanView ~ Irfan Skiljan
****
Available for: Windows Vista/2003 Server/XP/2000/ME/98/95
Download: home page | Download.com | FileHippo

XnView ~ XnView.com
***
Available for: Windows Vista/XP/2000/ME/98/95/3.x
Download: home page | FileHippo

:)

[Note]
* Google Picasa is also available with the free Google Pack.