Recently, I started using dwm as my main window manager. At less than 3000 lines of C, it’s somewhat more lightweight than others I’ve used in the past.
The automatic window placement and stacking is really handy, especially when you tend to have a lot of terminals open, it keeps things nice and organised. There is very little to configure, but I did modify the colourscheme a bit to make it darker. I also added some handy key bindings and a few wrapper scripts to allow the window manager to run in a loop so you can restart it without killing all your applications. My config.h and associated scripts are in subversion.