Blackbox is the window manager which revolutionized the desktop world. Throw away every assumption about window managers and start over from scratch. You get one, tiny desktop element called the toolbar which tells you workspace, time, and holds whatever programs you have minimized. You also have an element called the "slit" which is invisible unless you've loaded a dockapp into it. Right-clicking on the desktop gets you your root menu. Right-click on the elements gets you a small customization menu.
I'm done! That's all! Icons? What are those? Pixmaps in the menu? I think there's a plug-in for that. However, your root menu has a section called "styles", and therein lies one of the coolest features ever innovated. Blackbox can render gradients on the fly. So with plain, simple colors and font declarations, you can get an amazingly cool look!
There are also a couple of support programs, such as a background setter. But the general idea is to start with this minimal desktop, and download plug-ins to extend it into what you wanted.
The other beauty of Blackbox and the host of derivatives it has spawned is that every bit of customizing, from creating a theme to building the menu, is done from a simple text file using a very self-explanatory miniature scripting language. This makes it extraordinarily easy to write scripted programs to control it, and also lends itself to having a thriving development community.
Speed: Blackbox is super-fast! From the console, type "startx" and Blackbox pops into existence in an eye-blink. The desktop menu snaps to attention as soon as you click to summon it. Your programs never ran better, as Blackbox only uses a tiny amount of resources for itself. If one needs further proof of how light-wight Blackbox is, consider that Damn Small Linux, the distro that is dedicated to never exceeding 50MB of space, uses Blackbox for it's sole desktop.
I wouldn't recommend Blackbox for a green newbie - while there isn't much to learn (a manpage or two pretty much covers it), it is intimidatingly simple. Many may also see Blackbox as unusually Spartan. You want features, even simple things like keyboard customization, you'll have to download them.
But without reservation, I can recommend Blackbox for users of old hardware. Blackbox runs feather-light, and can bring new life to a system that was sluggish. But most of all, I consider Blackbox ideal for the software developer. The whole environment just screams "hack!", and there's even a derivative called - what else? - "Hackedbox"! The most popular derivative, however, is the mighty Fluxbox.