Subpixel morphological antialiasing

The best antialiasing currently available for Unity if you're using deferred shading and it's completely free and open source. Created by Thomas Hourdel.

You can download it from Thomas' github repository, here:
https://github.com/Chman/SMAA

Here's the official Unity forum thread if you have any questions about it:
http://forum.unity3d.com/threads/free-subpixel-morphological-antialiasing-smaa.323572

Thomas also has a number of really high quality assets on the Unity asset store worth checking out:
http://assetstore.unity.com/

Note: Just open the two images in new tabs to more easily compare them and see the difference between antialiasing on and off.