GIMP is simply t
he best freeware alternative to Adobe Photoshop. It’s an open source image editor, very powerful and quite stable.

GIMPFotografix has generated quite a bit of buzz. It's an excellent lightweight Photoshop alternative, with features like layers, masks, filters, scripts, and editable type.
FotografixPaint.NET is a very good choice for mid-level photo retouching because of its
excellent design and attractive interface. It also has options and
tools comparable to commercial software, but it requires you to install
Microsoft .NET Framework 2.0 or newer.
Paint.NETPhotoFiltre is a complete image editing and effects package, that will
not only amaze you with features but also the fact that it is free (for
personal use). It comes with many features that rival some commercial
packages, as well as additional add-ons that can be downloaded from the
web site.
PhotoFiltrePhotoscape is a complete photo editing suite that includes image
browsing/viewing, photo editing, picture page printing, photo resizing
and renaming, and even RAW conversion. The photo editor includes
standard editing features as well as automatic and custom adjustment
options for brightness, contrast, exposure, backlight corrections and
more. It also offers a variety of image effect filters and a collection
of image frames and masks that can be applied to enhance the
presentation of your photos.
Photoscape