GXT Widget Expander
If you've used GXT's Grid and RowExpander plugin, you know what I'm talking about. The current design lacks the ability to put arbitrary widgets into the expanded row. XTemplate is great but it falls short when you want richer user interaction inside the expanded row (something like Google Reader's grid).
At my work I created a modified RowExpander (I call it WidgetExpander) to tackle this limitation and it has been working great. So I created a project on Github (http://github.com/kevinjqiu/gxt-widget-expander) to open source this part. Hopefully you'll find this useful as I did.