Don't recreate standard features.
OS X comes with built in calendaring, a system-wide address book, a keychain for storing secrets, and a variety of other standard APIs. Use these, rather than rolling your own, and your program will work with all of the user's other programs. Opera on OS X uses the keychain, so all of your Safari passwords work if you switch to Opera. FireFox uses its own, so users have to manually transfer them over. This makes it a lot easier for users to switch to Opera than to FireFox, and to switch back if they don't like it.
I've expanded this tip, as I will do each Friday in January, into a full article: Cocoa Tips: Don't Reimplement Standard Functionality.
For more tips, please see my new book,
Cocoa Programming Developer's Handbook,
available in print and electronic formats.
Take advantage of special member promotions, everyday discounts, quick access to saved content, and more! Join Today.