I recently pushed LJSelectionView to GitHub. The project makes it easy to manage an NSView with a collection of subviews and their selection -- either by mouse clicks or "drag to select" actions. Selection rectangles, highlighting and selection management is something I've had to write more than once for Cocoa apps so I decided to write a standalone version to share.
I've set this up as a controller with several NSView subclasses. The controller is responsible for managing selection (and undo/redo) and the views are fairly dumb affairs that mostly just draw themselves (just how it should be). The exception is the main LJSelectionView that also understands the difference between selection, highlighting and your "content" views and has methods for managing them.
The two other views, LJSelectionItemView and LJSelectionRectView are configurable (up to a point) for line color, line width, fill, etc to change what the selection and highlighting rectangles (the stuff drawn around selected views) look like. If you want to go crazy with their appearance they're easy enough to modify or subclass.
For the ARC-shy (not that there aren't good reasons to avoid ARC at this point) all files will support ARC or non-ARC project automatically. It cluttered up the code a bit but I thought it was better than supporting one vs the other.
The selection behavior is the same as Adobe Illustrator: If you hold the shift key while clicking or dragging, you toggle whatever is being selected against the current selection. If you're not holding the shift key, whatever is selected always replaces the current selection. As it is now, views aren't selected until after the selection operation is completed. It shouldn't be that hard to change it to support "live" selection but again I'm following Illustrator here.
To use, just copy the four main classes and their headers (what's in the root of the GitHub repo) to your project. You'll also want to setup the view hierarchy in IB and make sure the correct outlets are connected. It is possible to setup the view hierarchy without IB you just have to make sure the right connections are made. You can look at the demo project to see how it's setup. The tests aren't automated in any way but you can run them in Xcode with ⌘-U as normal.
The whole thing is MIT licensed so do with it what you will.