Once a lightweight alternative, but today a very feature rich and quite complete desktop environment, Xfce is one of the older contenders. Its development started in 1996, then as part of the Common Desktop Environment. The project later took its own part, renamed as XForms Common Environment, thence the name. Today Xfce does not use XForms, and the acronym lost its meaning.
Xfce also uses the GTK2 toolkit and offers a desktop experience visually similar to old GNOME2. Although some might argue this, Xfce is still a relatively lightweight DE, meaning it's light on resource usage and runs fine on older hardware. Some might say it became bloated, but by that, they mean the plethora of added features and functionality. Such critiques do not only miss the point of what lightweight is supposed to mean if Xfce’s context, but completely miss the DE’s modular approach: Xfce’s components are packaged separately, making them standalone modules which might or might not get included in the final DE.
A great example of this is Debian’s own “minimal approach" towards its initial Xfce install: When you choose the Xfce desktop, you get little more than the basics with it, making it lightweight in both senses. Xfce was the “standard” desktop of Debian, before GNOME took over in the previous release (Debian 8 Jessie), and is still available as one of the main DE choices.