AquaSnap

Quickly Arrange Windows On Your Desktop
AquaSnap enhances the way you can arrange windows on your Desktop. Instead of manually sizing and arranging multiple windows, you can simply drop them onto specific hotspots (corners and edges) to quickly snap and size them into place.
Drag a window to the border of your desktop, and this window will be snapped and resized to fit one half of the screen. Drag this window to a corner, it will be snapped to fit one quarter. In addition, you can simply shake a window and it will stay always on top.
Other features include AquaMagnet (align windows like magnets), AquaGlass (make windows transparent while you move them) and AquaStretch to quickly size windows to full width or height.
The free version is limited to personal use and does not support multiple monitor configurations.