You might be missing compizconfig-settings-manager if you use Unity/Compiz.

And then have a look at the snap plugin.

• Snap Type Here you can define what types of snapping are available.

  
  
  
  
  
  • Checking Edge Resistance makes windows snap to edges and you must move the mouse further before they un-snap
  
  
  • Checking Edge Attraction makes windows snap to edges as you get close to them
  
  



• Edges allows you to define what is an edge

  
  
  
  
  
  • Checking Screen Edges makes windows snap to the edges of your screen
  
  
  • Checking Window Edges makes windows snap to the edges of other windows
  
  



• The value Edge Resistance Distance defines how many pixels space you must move your mouse before the window un-snaps




• The value Edge Attraction Distance defines how many pixels space windows must be next to each other before they snap.