Sparkle is an easy-to-use software update framework for Cocoa developers.
This little framework’s got a lot of features packed inside! Check ‘em out:

• True self-updating–no work required from the user.
• Uses appcasts for release information.
• Displays release notes to the user via WebKit.
• The user can choose to automatically download and install all updates.
• Displays a detailed progress window to the user.
• Seamless integration—there’s no mention of Sparkle; your icons and app name are used.
• Supports authentication for installing in secure locations.
• Supports DSA signatures for ultra-secure updates.
• Really, really easy to install.
• Sparkle requires no code in your app, so it’s trivial to upgrade or remove the module.

Get it here