If free software development goes by trends, then the current era might be called the Age of Extensions. In the last few years, every application from the Mozilla family to OpenOffice.org to Gedit has created frameworks in which developers can add their own small bits of functionality to an application. In the last 10 months or so, a community has taken this trend directly to the desktop with what it calls "screenlets" -- small applications that are added directly to the desktop. The result is dozens of tools, some new and many old, that are in most cases not only themable, but also heavily customizable.
Screenlets.org, the site for their development and listing, describes screenlets as small Python applications, and the screenlet libraries as an effort "to simplify the creation of fully themeable mini-apps that each solve basic desktop-work-related needs and generally improve the usability and eye-candy" of the modern desktop. They are specifically intended for composited window managers, but most of them also work with 2-D window managers such as Metacity.