Ripplr is a Twitter-like application based on the Hubub project that was used in the Grails In Action book published by Manning Publications.
I created it as I followed along in the book and later converted it into a Grails 3 project, but there is still a lot of work that could be done.
I'll get around to those updates at some point and highlight them via blog posts on my blog http://joelforjava.co.