Is there are a 2d engine like unity but more focused on creating 2D Games?
You can use Unity to create 2D games, you would just not make use of the Z axis (unless you want a background). You want to change the camera to orthographic. That why you it will be a front facing camera.
Now if you want to use 2d images (instead of 3d images that you only see the side of) you can use spriteManager a free plugin, spriteManager2, or 2d Toolkit (both not free, but good value for what you get)
Here is a link to get you started on spriteManager… 1
Sp00ks is right, however there are are quite a few 2D-oriented engines.
Adobe Flash: http://www.adobe.com/products/flashplayer/
Torque: http://www.garagegames.com/
Corona: Anscamobile.com
Gamesalad: http://gamesalad.com/