I'm not sure I'll call xmonad popular. Also I know quite a lot of people who hate it -- because it's configuration language is "write some Haskell and recompile", so it tends to be very fragile (people who use Haskell love it)

Also found it crashed a lot, which I would have hoped would be one of the things Haskell would solve (the problems revolved it having to integrate into C code for linking to X, but that's part of living in the real world).