I think, there is no tool similar to Xermes avilable. W3C XForms known implementations are listed down (Italics = quotations from homepage). Note that there (i.e. anywhere) is no tool, able to generate GUI from the application logic. If I'm mistaken, pleas correct me!

GUI frameworks:

JioMask provides a declarative way of defining user interfaces including forms for data input and tables or tree-like structures for data output. Currently, it is already possible to define self-validating input forms. The input forms are described using W3C XForms. From these descriptions implementation dependent code (Java, XML/XSLT/XSP, HTML) is generated that displays the user interface and provides convinient developer access to the data of the form(s). JioMask is interesting piece of software. If you want to write your own XForms documents and have them converted to Swing-enabled Java classes, use it! But if you need forms generated "on the fly", directly from the model, JioMask is not sufficient.

Chiba is a Java implementation of the W3C XForms standard for use in web-applications. The goal is to provide a production quality processor to handle form-processing generically. As you can see, Chiba is root and branch Web-based and it's impossible to use it in desktop applications. But if you are searching for this, feel free use it!

ExFormula is an implementation of the XForms specification. It allows you to define XML Models and Instances associated with it, to process dynamic forms based on an end markup. There is not a lot of info on the homesite and the founder claimed ExFormula dead. Thanks to Cybernd, jXForms maintainer (see Dependencies) for this info.

XML WebAccess - well known, stable, robust Web-based XForms implementation. Commercial.

XFE, a forms engine developed by E-XMLMedia, radically simplifies development of forms based data entry systems. A one-page interaction document, written by a web designer in XML, defines the form, describes the desired interaction with the user, and specifies the information desired from the user. The form definition is given to XFE. XFE manages multiple rounds of interaction with the user through the user's favorite browser. The result of data entry is delivered to the web application as a validated XML document. Web-based, commercial.

XForm is a Java package that dynamically generates and validates web forms. It is Free Software. XForm allows the servlet developer to treat web forms as a black box. The form, it's validation, and error behavior are specified by a single XML Document. User data is returned to your application as an XML Document object for further processing.

There are also frameworks not based on XForms, and there is also another "standard" called XUL, managed by Mozilla as an alternative to XForms.

If you know an another tool, that should be listed here, please drop a line!