[callahad]

There are a bunch of apps that can do this for you, with varying degrees of automation:

ShiftIt (Free/OSS): https://github.com/fikovnik/ShiftIt/tree/shiftit16

DoublePane ($3): http://5amcode.com/

Tyler ($10): http://blog.reflare.com/tagged/tyler

SizeUp ($13): http://www.irradiatedsoftware.com/sizeup/

Divvy ($14): http://mizage.com/divvy/

There are some old versions of ShiftIt available in the GitHub downloads, but you might want to compile a more recent version yourself. It's what I use on my 11" MacBook Air, where I mostly use the left-half, right-half, full-screen, and center-window commands. If your use case would be similar, DoublePane might be an easier choice, though it doesn't look like it does centering.

[stephth]

Optimal Layout (Application switcher + window manager): http://most-advantageous.com/optimal-layout

I just stumbled into it, I recommend watching the video.

[moe]

And they all make you really sad if you're looking for a real tiling wm or, worse, have actually used one before.

The window manager situation on OSX is still dire.