If you right click on the maximize button of a window, it will maximize the window only horizontally. Likewise if you middle click, the window will get maximized only vertically.
Bonus: press and hold the Alt key and Click+Drag a window to move it.
I use Compiz, I don't know if this might work without it..
I did not know about maximizing windows with other than LMB, thanks for it .) Looks like all of mentioned features are compiz-independent (I use openbox).