Yes, i've found ocp quite recently. It looks promising. I've also found several more on Atari mania like FUD editor (three versions) and art director. There is also STOS sprite editor. Anyone knows about other ones?
From practical work with pixel artists myself, I know they prefer regular paint tools like Neochrome or Degas over dedicated sprite editors.
Why is that? Anyone can tell something about this? In degas there no possibility to preview animations, so this isn't very convenient. They also cannot produce data in some sane format (with additional animation data, sprite size/positions etc..).