GIMP is a free and open source image editor. Just like Adobe Photoshop, GIMP is capable of performing variety of image manipulation tasks and it supports plug-ins and extensions so possibilities are unlimited with this free image editing software.
GIMP 2.8.4 for Windows, Linux and OS X including more responsive drawing with the brush outline tool, plus better names for the default filters when saving or exporting.
Download GIMP for Windows
Release Notes
Update: Gimp 2.8.4 Portable version released!