I recently changed my launcher from the default TouchWiz (I have an AT&T SGS3, with the official 4.1.1 installed) to Nova Launcher. I enjoy Nova, but I've noticed something strange: the Calendar and Weather widgets that were available under TouchWiz aren't there anymore. They're still there in TouchWiz (Home Switcher makes it easy to switch back and forth). What's going on?
The widgets referred to in the question are TouchWiz-specific widgets. They will only show up in TouchWiz because they don't use the standard Android widget interface. I've found some references to Samsung releasing a TouchWiz SDK to develop such widgets, but those articles are from 2009 and the links they contain now lead to Samsung's developer home page, rather than to a specific page. I've also seen references to some of the widgets being ported to work on any Android phone, but I can't confirm as XDA seems to be down at the moment.