I had a poll like this sometime ago, difficult to get many votes for something other than KDE/Gnome.
Some of the options are not desktop environments, but simple window managers, so anyway I will name my favourites from both areas.
FVWM has to be the greatest, fastest, most powerful window manager in existance. Every bit of it can be customised to make it look or feel like MVM, Windows 95, Mac, or anything you can imagine. You can customise the application window buttons, hey bindings, specify powerful functions to perform complex tasks, and what not. The problem is that configuring it is not so easy, and it can take quite a while. But you do have many sample config files available, which will help get you started quickly.
IceWM follows next in line, it is not as configurable as FVWM, but it has quite a decent out of the box look and feel, it provides a nice integrated taskbar, with a windows 98 look (menu, quick launch items, taskbar, clock), so it makes it good for people migrating from windows. Currently I am back to IceWM because I am facing certain problems with getting a net monitor to work on FVWM.
Blackbox is another nice one, switching between applications may be a little awkward at first, if like me, you maximise every open window. But on undersanding the little controls on its taskbar, it can be a very efficient and fast window manager.
Coming to desktop environments,
Although KDE is quite fast, I prefer XFCE to it for its simple elegance. It looks quite nice and provides a light alternative to Gnome.
EDE (Equinox Desktop Environment) is a very fast EFLTK based desktop, it resembles windows 9x in appearance and provides many good GUI tools for frequently used purposes.