Sparkle

is an easy-to-use software update framework for Cocoa developers.

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

New in Sparkle 1.5:

Optionally sends user demographic information to the server when checking for updates.
Dual-mode garbage collection support for any memory management style.
Sparkle doesn't bug the user until second launch for better first impressions.
Sparkle can install .pkg files for more complicated products.
Supports bundles, preference panes, plugins, and other non-.app software.
Supports branches due to minimum OS version requirements.
Deep delegate support to make Sparkle work exactly as you need.
Tons of other stuff! Read more about what's new.