Its my understanding that the Viewer will be free. Much the same way as the Adobe reader for PDF files. To build a world you would need the Atmosphere Builder. Avatars however can be made in other applications and then loaded in the Viewer.

Currently you may do better running the Viewer on its own, the one I have embedded in the web page is not currently allowing me to type into the text field for some reason.

I am indeed very excided by all this :)