UPDATE I have not used these scripts in a while and I do not support them anymore. I use the excellent Moom, which I recommend.
A collection of Applescripts which resize and tile windows neatly over the desktop. They are best used by assigning them a keyboard shortcut, through FastScripts for example.
I personally use:
Maximize Vertically ⌃` Left ⌃⌥⌘← Right ⌃⌥⌘→ Top ⌃⌥⌘↑ Bottom ⌃⌥⌘↓ Upper Left ⌃⌥⇧← Lower Left ⌃⌥⇧↓ Lower Right ⌃⌥⇧→ Upper Right ⌃⌥⇧↑ Left Third ⌃⌥← Right Third ⌃⌥→ Upper Center Third ⌃⌥↑ Lower Center Third ⌃⌥↓ Center Third (none)