I have given training in animation and have used it in the classroom for years now, but was introduced to Sam Animation for the first time by Oscar Stringer today at #BETT2011.
The software from www.samanimation.com/ is cross platform. It starts from the great price of FREE.
Although the interface is a little unpolished and quirky, it does the job of capturing animation frames well enough. Well worth giving it a try if funds do not stretch to the likes of iStopMotion, I Can Animate or Zu3D.