If you are looking for an easy and free tool to create animations.
Animeeple is an easy to use character animation tool.
You can easily import and animate characters, layer and sequence them , edit and modify animations (with booth FK, IK handles ).
It even allows placing a camera and render out your scenes as video. But for most purposes you'll export characters and/or animations using FBX, Collada, BVH and XML.
I sucessfully managed to get my animated character imported into XSI Mod Tool (using FBX) and Blender (using Collada).
And you don't have to start from scratch. Animeeple offers a marketplace where you can download for free or buy additional content (animations and characters). Right now they offer several characters you can animate and over 100 free animations to start with. On their marketplace you can upload your own characters and animations and sell them (they take a 25% comission).
One more highlight is Animeeple's feature to animate your Second Life avatar. You can download your SL avatar into Animeeple and then create your animations with it. Then upload it inside this tool or export as BVH and upload via you SL client. Their site offers a some video tutorials as well.