FastStone Image Viewer - FastStone Image Viewer is a fast, stable, user-friendly image browser, converter and editor.
Aseprite - Aseprite is an art program dedicated to the creation of pixel art.
IrfanView - IrfanView ... one of the most popular viewers worldwide.
Pixel Art to CSS - Draw and animate pixel art, export the results to CSS
Photoscape - high quality photo editing software, that enables you to fix and enhance photos.
Trixel - Instagram for triangular pixel art