SwellRT is a platform to develop collaborative apps based on Apache Wave. Currently provide a client for Web but it can be ported to Android/Java
In a nutshell, SwellRT provides an API to work with objects that can be shared and modified by different participants in real-time, transparent storage and eventual consistency.
In particular, it support collaborative real-time editing of text, including an extensible text editor for Web.
SwellRT servers can be federated.