One of the compelling features in Rails is the ease with which forms for creating/editting/updating/deleteting objects can be created. Tapestry has a lot of power under the hood with respect to forms, but it still doesn't come cheap enough out of the box. A centerpiece of Trails is a component that builds a full form for editting an arbitrary object. This idea has resurfaced as a new standalone component, BeanForm.
Just plug the following into your page's HTML template:
<span jwcid="@bf:BeanForm" bean="ognl:pojo" save="listener:save" delete="listener:delete"/>
BeanForm will build a complete form from this, adapting to each individual property's type. If you are using EJB3 or Hibernate annotations, BeanForm will pick up those annotations to build out appropriate client- and server-side validations.
And its extremely extensible and customizable even beyond that. Cudos to Daniel Gredler for putting this together.