aero snap was one of the launch features of Windows 7 that was quickly copied into other desktop systems (most recently, gnome 3). I've written about it before. Essentially, dragging windows to hot regions of the screen resizes those windows to occupy some area of the screen. To the top: the whole window area (maximize). To the left or right, 50% of the width of the window area, all of the height.
This is a pretty useful feature that I wanted to steal for myself. There is a somewhat fragile-looking (but functional) way of achieving this using compiz and some embedded shell scripts which I used for a while, but has several drawbacks: there's no visual indication that resizing will occur; resizing occurs after a time interval and not in response to you releasing the window from its drag; it's therefore very easy to accidentally resize windows (focussing a window adjacent to an edge and parking the mouse pointer in the edge region is sufficient).
Aero snap is not without its own shortcomings: whilst having side-by-side windows is great for comparing two documents, you can't arrange two windows top-to-bottom, each occupying half of the vertical height of the screen. This is very useful for working on two spreadsheets.