Can I use a JSF 2.0 implemntation on a J2ee 1.4 server ( jboss 4.0.5 / 5.0 , Weblogic 9, WebSphere 6.1 )?
Are there any (free) implementation that runs well on all platforms?
No, that's not possible, because JSF 2.0 requires version 2.5 of the Servlet API, and J2EE 1.4 only supports Servlet 2.4. However, it is possible that a given concrete J2EE 1.4 implementation may support Servlet 2.5 anyway (but lack other components of Java EE 5).
I'm using Apache MyFaces and have no complaints, however I am not using it cross-platform. But I wouldn't expect issues in that regard; JSF is really just a framework built on top of the Servlet API and doesn't do anything remotely platform-specific.