scrotwm on OS X
I’ve posted briefly about the tiling window manager scrotwm before, but forgot to mention that it works very well on OS X.

Here’s how to install and set it up.
First, download:
cvs -d anoncvs@opensource.conformal.com:/anoncvs/scrotwm co scrotwm
Then change into the osx directory and do the usual:
cd ./scrotwm/osx
make
sudo make install
Next, stick the following in ~/.xinitrc
xmodmap ~/.xmodmap
exec /usr/local/bin/scrotwm
And create that ~/.xmodmap file so you can use the left Alt key to control scrotwm:
clear Mod1
keycode 66 = Meta_L
add Mod1 = Meta_L
That’s it, really: fire up X11 and it’ll start scrotwm and an xterm. Check the man page for a list of keybindings; when you want to get out of X11, whack Alt + Command + A.
Optional Extras:
You’ll probably want to tinker with your ~/.scrotwm.conf, though the defaults are pretty sensible. Mine is just two lines:
color_focus = black
bar_enabled = 0
To make xterm look pretty, you can use an ~/.Xdefaults file (mine is here). feh will set a desktop picture. And if you want a status bar—to keep track of which workspace you’re in, &c.—scrotwm is designed to work with dmenu.