GitHub have launched a desktop client for Mac OS X called simply GitHub for Mac. Although there are several desktop Git clients for Mac already (Git Tower, GitX and others), the popularity of GitHub generally is likely to see a surge in interest to this new product.
The application itself is implemented in Objective-C, but uses the objective-git wrapper for libgit2. However, the user interface uses Chameleon, which is a UIKit port to OSX. This is the same library that is used to implement the Twitterific application. Although it's unlikely that it would make sense, this opens the door to GitHub for Mac being ported to GitHub for iPad in the future.
GitHub for Mac aims to make it easier to clone a repository from the website with the "Clone in Mac" button, create a fork, commit and upload changes.