In the past I went through about ten window managers on my desktop (I remember at least fvwm, fvwm95, sawfish, sawfish2 and now finally blackbox). For several commercial projects we use also desktop environments like KDE and GNOME. I even tried to use KDE on my system, but I prefer using RAM for work not for fun...
But sometimes you are faced with a problem that is outside your current knowledge and you need to look for something new, investigate new software etc.
This is why I'm spending a lot of time with qvwm. It is a window manager that is a parody to Microsoft Windows 98 desktop. A nice parody: icons are animated (it is unusable for real work in fact), some icons are vertically mirrored compared to the Windows systems (nice ;-) and it is possible to customize it to death (see e.g. this screenshot). And this is the main advantage why I'm using it in this project. The second most important advantage is its license: GNU GPL.
Try it - it is quite old, but impressive anyway.-----