There are currently a million ways to embed distinct snippets of content and functionality on web pages. The most common two are probably iframes and some form of javascript syndication (you put this script on the page, we'll document.write some stuff on your page). In the XML world, there's RSS, Atom, and the WSRP Portlet spec for exchanging small pieces of data (although it could be said that neither RSS or Atom are meant for this sort of thing). WSRP is the only spec that's specifically meant for exchanging widgets (or modules or portlets), and it's extremely complex.
I (User:KevinLawver) think this is a perfect opportunity to create a microformat for creating, delivering and discovery of small (micro, even) content on the web.
