Other popular DEs and WMs

LXDE and LXQT

LXDE, meaning Lightweight X11 Desktop Environment had been around since 2006, took "light-weight" to the next level. Even compared to XFCE, LXDE is super-fast, resource efficient and basically bare bones. Of course one can argue that some WMs are even lighter, but that would be beyond the point. LXDE is a Desktop Environment, with the additional bells and whistles WMs might miss (although as few of them as possible).

LXDE is as fast as it is ugly

LXQt is what LXDE is becoming. While LXDE was written in GTK2, the original developer, (who started the project in 2006) did not like GTK3 and began porting the DE into the Qt toolkit. Later on, the project merged with Razor-qt, another lightweight, Qt-based DE, and so LXQt was born.

Vanilla LXQt is even uglier, but it can be tweaked into prettiness

Although there is still some confusion about which version each distro is currently using, LXQt will eventually completely replace LXDE, or at least that is the plan. Possibly one major obstacle would be to find a long name, of which LXQt can be an abbreviation of.

Enlightened Desktop?

Enlightenment (E) is not only for the spiritually minded. An old, weird-looking Desktop environment or WM (some say it's both, some say it's neither, others say it's in between) bears the same name so that unenlightened people can make use of enlightenment too. (It would be difficult to even imagine what would happen if enlightened people began using it.)

90s aesthetics nostalgia. e17 is outdated, yet surprisingly flexible

Enlightenment 0.16 (E16) had been around since 1997, making it the one of the oldest that's still in everyday use and active development. While the next version (0.17, or E17) took 12 years to develop (from 2000 to 2012, not because it was so difficult, or polished, but simply because nobody cared to work on it for many years), development today continues with new versions coming out regularly.

Enlightenment was the original "eye candy" desktop, with compositing, and fancy effects, and today it's still a proof of concept that fancy animations and shadows don't need a lot of resources. Configured right, E can be as minimalistic as the simplest WM (running on approx 20MB or memory), while offering animations and compositing effects that would please anyone who admires late 90's aesthetics.