Shamans and Witches often get confused with each other on a few levels.
A shaman typically has a closer connection to the natural world than say a witch does. A shaman's magic then will appear to be more in harmony with the rest of the world.
Along with witches, Shamans are also often a forgotten source of magic in many Fantasy Roll-Playing games.
When I was working on my book of Witches for the d20 game, there was a group doing a d20 Shaman. I worked with them and they with me and it was great to be able to exchange ideas.
Now I am doing "Strange Brew: The Ultimate Witch & Warlock" and as it turns out the next book inline is "Strange Brew: The Ultimate Shaman" and again these two teams will be sharing ideas. I enjoy this, it it nice to have people to bounce ideas off of.
I don't recall ever playing a Shaman character though. Any time it would have come up I played a Druid or a Witch. I might have to give that a go sometime.