ActiveForm provides a DSL for defining complete XHTML forms with validation. You can compose your own widgets and (nested) form sections from basic elements. Markup is generated by Builder, using several wrappers and callbacks which render their child elements accordingly. This allows one to customize all output.
Both serverside as well as clientside validation is supported and values are bound in a nested fashion to their elements, similar to how Rails expects params from forms.
It’s possible to output elements, sections and the complete forms, allowing easy integration with ERB templates. While it’s distributed as a plugin for Rails it can easily be used in standalone Ruby scripts.