This is improved MCImage class that originally was wrote by Drew McCormack from The Mental Faculty.
I decided to improve it and make it a little smarty than it was.
Now it correctly stretches whole center image area (not only 1x1 center area) and supports iOS5-like stretching.

How to use:
1. Like in iOS, load image from file and make it stretchable:
NSImage *stretchableImage = [[NSImage imageNamed:@"myImage.png"] stretchableImageWithLeftCapWidth:10 topCapHeight:5];
2. Assign it to NSImageView and it will draw it as you expect:
myImageView.image = stretchableImage;

P.S. There is also iOS5-like mathod available:
BFEdgeInsets insets = BFEdgeInsetsMake(5, 8, 2, 8);
NSImage *stretchableImage = [[NSImage imageNamed:@"myImage.png"] stretchableImageWithEdgeInsets:insets];
myImageView.image = stretchableImage;