RightZoom is what you want (scroll down to the bottom of the page or use this MacUpdate link)
This free tool fixes an inconvenient behavior of green Zoom button in Mac OS X. It will always expand windows on a full screen instead of strange resizing.
Another one is BetterTouchTool. You might not think of BetterTouchTool as being a window manager, but it has window snapping (as seen in Windows 7). Take a look:
Cinch also does the same thing, but it costs $7.