Sumo Paint

A powerful online image editor that mimics the popular Photoshop. It can be loaded within your browser and you can use its features instantly. Allows you to save your PNG/GIF images afterwards or load/import existing work and provides a wide variety of tools for editing as well as pixel art.

Piskel

Piskel, free online sprite editor. A simple web-based tool for Spriting and Pixel art. Create pixel art, game sprites and animated GIFs. Freeand open-source.

Color Ramp Creator

A very useful website that automatically creates and adjusts a 4, 8 or 16 shade color pallete for pixel art.

by KayTrance

Graphics Gale

Graphics Gale is a lot more approachable and easy to use, and it comes for a price of around $20, which isn't too bad.

That said, the trial version isn't time limited, and comes with enough options to make perfectly good graphics. The only thing it leaves out is .gif image management, which isn't much of a problem, since .png is better anyway.