As most modern developers know, a great way to save time when building an application is to stand on the shoulders of others, by making use of open-source/free software that is all around. You see if somebody else has already solved your problems in a general way, and incorporate their software libraries into your application. Ideally, you give a little back to the community and perhaps contribute something of your own for others to use.
I wanted to highlight a great Cocoa UI addition — or rather, replacement — that we are using in our forthcoming application. It's RBSplitView, a replacement for the not-keeping-up-with-the-times NSSplitView class. Its developer, Rainer, has been extremely helpful by making this class available, and responding to feature requests to enable developers to make more "modern" software. If you want a better split view, check this class out!