DotWeb is a PHP library for handling HTML controls which is very similar to .NETs Webforms. Visit http://www.gotmono.com/docs/aspnet/htmlcontrols.html for more information.
trillbox is a flexible and extendable toolkit for building dynamic web pages. Written in Perl and based on Template::Recall, it provides "widgets" (or controls) that you can quickly integrate into your Perl web application.
Pieforms provides a simple, unified way to create, validate and process forms all with a common look and feel, with support for pluggable elements, renderers and validation rules. Supports AJAX submission, date picker/other advanced controls. PHP5+.
...It is written in C# any MsNet and Mono. Provides abstract data layer for keys and countries, globalized WebControls with multiple assemblies, Language Switcher, Money, Section 508 controls and much more
hostingstar.com.au's open source software.
To be used primarily for dotnetnuke modules, a cross platform backup system, .net (dotnet) user controls, 3d plugins and many other bits and pieces.