I don't run xmobar or trayer...

It's nice that we don't need to write these things in C to be small and fast.

Me neither and I can't work without XMonad.Prompt, especially the Shell instance, for spawning executables. I don't remember Tuumo's ion in detail but the only other launcher that behaves similarly in a sane way is cwm's built-in menu. dmenu_run doesn't work the right way and is limited in what it launches or auto completes. Like ion there are also instances for ssh and man pages. When using the shell prompt the memory usage goes up by 1 or 2 megabytes but that's constant. Any time I try spectrwm or dwm I always come back to xmonad.

Trayer too (if you use it). I'll have to take a look when I get home to see how it all adds up.

You can try taffybar [1] but it's a bit more involved (I don't think it's packaged anywhere). Has both widgets, task bar and tray.

1: http://hackage.haskell.org/package/taffybar